题记
感觉说的挺好的，值得学习 1 版权声明：本文为博主原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接和本声明。
本文链接：https://blog.csdn.net/liujiaqi12345/article/details/88357041
Leetcode JAVA 题解： https://github.com/mJackie/leetcode
自己日常刷题经过是这样的： 拿到题目，看一眼Difficulty，然后自己思考一下解题思路。如果解不出来，就记下在哪里卡住了，难点在哪。
如果对应的题目有Solution，就看Solution，没有的话就点Discuss，按Most Votes排序，看排名最高的解法。
对比一下自己的解法与最优的解法的差别，总结一下为什么没想起来，记录下来这个思考的过程。
关掉别人的代码，开始Coding，Debug，Submit。
附上自己总结的几条经验： 先刷两个Top专题。Leetcode 上有个List选项，里边有两个专题，分别是Top 100 Liked Questions和Top Interview Questions。这两个List中有很多重复的题，加起来一共150道左右。都是经典的题目，将这150道刷完基本上所有的题型都见过了，而且多数经典题目都会涉及，是提升最快的一个方法。 注意记录、总结与复习。自己写过的代码一定要保存下来，刷题的时候也要记下主要思路和注意点，这样在复习的时候也能对比发现自己哪里还能改进，之前犯得错误有没有重犯。可以将相互关联的题目对比着一起看，方便总结与记忆。一定要时常复习刷过的题，复习比一味的追求数量更重要。 做好Easy，没必要死扣Hard。LeetCode上很多Easy的题目看似简单，实则想要写出Perfect的代码并非易事。多思考如何优化Easy，Medium的解法实际上比花精力解Hard题更能提高自己。况且面试的时候Hard被问的概率太小了。 切忌眼高手低。不要想着自己知道思路解法了就是会了，一定要亲自Coding，手撸出来。我在刷的过程中就经常在Debug的时候才发现自己忘记考虑了某些条件。不把代码写出来，只看别人的答案对自己是没有多大的提高的，只有亲自AC了题目，才能算做过一道题。
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版权声明：本文为CSDN博主「Jackie.Liu」的原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/liujiaqi12345/article/details/88357041
还有这注释方式也不错，学习
语言: Java
-
说明： 每道题在代码头部都添加了我的解题思路和批注，Eg:
/*****
* 287. Find the Duplicate Number
* 题意：n+1个数属于[1~n]，找出重复的那个数
* 难度：Medium
* 分类：Array, Two Pointers, Binary Search
* 思路：如果nums[i]不在对应位置，则和对应位置交换。如果对应位置上也为该数，说明这个数就是重复的数字。这个方法改变了数组。是错误的。
* 另一种方法，把问题转换成有环链表，找环的起始节点。O(n) O(1) lc142
* 二分查找，每次看一边数字的个数, O(nlog(n)) O(1)
* Tips：剑指offer原题
*/
1. 两数之和
1 public int[] twoSum(int[] nums, int target) {
2 int[] result = new int[2];
3 Map<Integer, Integer> map = new HashMap<Integer, Integer>();
4 for (int i = 0; i < nums.length; i++) {
5 if (map.containsKey(target - nums[i])) {
6 result[1] = i;
7 result[0] = map.get(target - nums[i]);
8 return result;
9 }
10 map.put(nums[i], i);
11 }
12 return result;
13
14 }
15
2. Add Two Numbers
/**
* Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8 Explanation: 342 +
* 465 = 807
*
* 题意：对于俩个链表。对应节点相加，满十进一
* 思路：先判断对应节点是否至少存在一个有值，有则相加，然后移动节点向下，循环如此，如果说最后一次相加，进位（carry）不为0，则要显示，其次，返回值要从返回链表的第二个几点开始
*
* @param l1
* @param l2
* @return
*/
public static ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode resultNode = new ListNode(0);
ListNode p = l1, q = l2, curr = resultNode;
int carry = 0;
while (p != null || q != null) {
int x = p != null ? p.val : 0;
int y = q != null ? q.val : 0;
int sum = x + y + carry;
carry = sum / 10;
curr.next = new ListNode(sum % 10);
curr = curr.next;
if (p != null) {
p = p.next;
}
if (q != null) {
q = q.next;
}
}
if (carry > 0) {
curr.next = new ListNode(carry);
}
return resultNode.next;
}
3. Longest Substring Without Repeating Characters
1 public static void main(String[] args) {
2 String s = "abbabc";
3 System.out.println(lengthOfLongestSubstring(s));
4 }
5
6 public static int lengthOfLongestSubstring(String s) {
7 int max = 0;
8 // ”记录当前重复字母的最新位置“
9 int j = 0;
10 HashMap<Character, Integer> resultMap = new HashMap<Character, Integer>();
11 for (int i = 0; i < s.length(); i++) {
12 if (resultMap.containsKey(s.charAt(i))) {
13 j = Math.max(resultMap.get(s.charAt(i)) + 1, j);
14 }
15 //”当前位置-上次重复的最大位置+1“
16 max = Math.max(i - j + 1, max);
17 resultMap.put(s.charAt(i), i);
18 }
19
20 return max;
21 }
7. 整数反转
1 public int reverse(int x) {
2 int ans = 0;
3 while (x != 0) {
4 int pop = x % 10;
5 if (ans > Integer.MAX_VALUE / 10 || (ans == Integer.MAX_VALUE / 10 && pop > 7))
6 return 0;
7 if (ans < Integer.MIN_VALUE / 10 || (ans == Integer.MIN_VALUE / 10 && pop < -8))
8 return 0;
9 ans = ans * 10 + pop;
10 x /= 10;
11 }
12 return ans;
13 }
14
8. String to Integer (atoi)
public static int myAtoi(String str) {
// 1字符串非空判断 ""||" "
if (str.isEmpty() || str.trim().isEmpty()) {
return 0;
}
int index = 0;
int sign = 1;
int total = 0;
//1检测第一个非空字符串是什么
while (str.charAt(index) == ' ' && index < str.length()) {
index++;
}
//1判断这个数是正数还是负数
if (str.charAt(index) == '+' || str.charAt(index) == '-') {
sign = str.charAt(index) == '+' ? 1 : -1;
index++;
}
//1判断是否是数字，是否越界，如果越界就取越界的边界值
while (index < str.length()) {
int digit = str.charAt(index) - '0';
if (digit < 0 || digit > 9) {
11. Container With Most Water
/**
* 解法1：俩边的边为起点，进行计算，如果左边的边比右边的小，左边第二条边和当前右边的边进行计算，如果右边的边小于左边的边，则右边的第二条便进行计算，依此类推
*
* @param height
* @return
*/
public static int maxArea(int[] height) {
int i = 0, j = height.length - 1, res = 0;
while (i < j) {
// ‘取最大值’
res = Math.max(res, Math.min(height[i], height[j]) * (j - i));
if (height[i] < height[j]) {
i++;
} else {
j--;
}
}
return res;
} /**
* 解法2 遍历所有的可能结果n(n-1)/2中情况
*
* @param height
* @return
*/
public int maxArea1(int[] height) {
int max = 0;
for (int i = 0; i < height.length; i++) {
for (int j = i + 1; j < height.length; j++) {
max = Math.max(max, Math.min(height[i], height[j]) * (j - i));
}
} return max;
}
17. Letter Combinations of a Phone Number
public static List<String> letterCombinations(String digits) {
List<String> ret = new ArrayList<String>();
Map<Character, String> map = new HashMap<Character, String>() {
{
put('2', "abc");
put('3', "def");
put('4', "ghi");
put('5', "jkl");
put('6', "mno");
put('7', "pqrs");
put('8', "tuv");
put('9', "wxyz");
}
};
//‘非空校验’
if (digits == null || "".equals(digits)) {
return ret;
}
dfs(digits, 0, "", map, ret);
return ret;
}
public static void dfs(String digits, int idx, String path, Map<Character, String> map, List<String> ret) {
if (digits.length() == path.length()) {
ret.add(path);
return;
}
//‘循环配合递归’
for (int i = idx; i < digits.length(); i++) {
for (char c : map.get(digits.charAt(i)).toCharArray()) {//这里是第个数字的对应的字母
dfs(digits, i + 1, path + c, map, ret);//这里进行递归，对应的第二个数字的循环，和第一个字母进行拼接
}
}
}
19. Remove Nth Node From End of List
public class ListNode {
int val;
ListNode next;
ListNode(int x) {
val = x;
}
/**
*‘本题思路：建立俩个链表，一个是dummy,复制原链表，另一个链表（first）为了计算链表长度；然后在用first链表指向dummy,删掉指定位置的元素’
*‘注意，应为是dummy指向head,所以多了一个节点，在指定删除位置时不用减一；另外返回时应该返回dummy.next，第一个节点是我们自己定义的’
* @param head
* @param n
* @return
*/
public static ListNode removeNthFromEnd(ListNode head, int n) {
ListNode dummy = new ListNode(0);
dummy.next = head;
ListNode first = head;
int length = 0;
while (first != null) {
length++;
first = first.next;
}
int position = length - n;
first = dummy;
while (position > 0) {
position--;
first = first.next;
}
first.next = first.next.next;
return dummy.next;
}
public static void main(String[] args) {
ListNode a1 = new ListNode(1);
ListNode a2 = new ListNode(2);
ListNode a3 = new ListNode(3);
ListNode a4 = new ListNode(4);
ListNode a5 = new ListNode(5);
a1.next = a2;
a2.next = a3;
a3.next = a4;
a4.next = a5;
ListNode a6 = removeNthFromEnd(a1, 2);
while (a6 != null) {
System.out.println(a6.val);
a6 = a6.next;
}
}
}
20. Valid Parentheses
/**
* 题意：括号匹配，俩个匹配的括号之间是不允许有为匹配（也就是单个的）括号
* 解题思路：通过入栈的形式，如果未匹配就入栈，匹配就出栈，最后如果栈不为空或者栈顶元素不当前元素不匹配就返回false
*
* @param s
* @return
*/
public static boolean isValid(String s) {
Stack<Character> stack = new Stack<Character>();
for (char c : s.toCharArray()) {
if (c == '(') {
stack.push(')');
} else if (c == '[') {
stack.push(']');
} else if (c == '{') {
stack.push('}');
} else if (stack.isEmpty() || stack.pop() != c) {
return false;
}
}
return stack.isEmpty();
} public static void main(String[] args) {
System.out.println(isValid("["));
}
21. Merge Two Sorted Lists
int val;
ListNode next; ListNode(int x) {
val = x;
} /**
* 本题思路：‘将当前节点l1.next和L2的当前节点（第一个节点）进行比较,如果小于等于（注意：等于也是可以的）,继续往下走，反之则进行节点替换（l1.next和l2进行替换），当l2为null时（也就是l1.next=null）结束循环’
* @param l1
* @param l2
* @return
*/
public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if (l1 == null) {
return l2;
}
if (l2 == null) {
return l1;
}
//此处是为了保证第一个节点时最小值
ListNode tmp;
if (l1.val > l2.val) {
tmp = l2;
l2 = l1;
l1 = tmp;
}
ListNode newListNode = l1;
while (l2 != null) {
//遍历节点进行组合
while (newListNode.next != null && newListNode.next.val <= l2.val) {
newListNode = newListNode.next;
}
//比较排序
tmp = newListNode.next;
newListNode.next = l2;
l2 = tmp;
}
return l1; } //展示当前链表的值
public static void sysoListNode(ListNode l1) {
while (l1 != null) {
System.out.format("%d->", l1.val);
l1 = l1.next;
}
System.out.println("===================");
} /**
* 大神的解法：原理和上面一样，只是利用递归的原理
* @param l1
* @param l2
* @return
*/
public static ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
if (l1 == null)
return l2;
if (l2 == null)
return l1;
if (l1.val < l2.val) {
l1.next = mergeTwoLists(l1.next, l2);
//此处时为了更直观看当前链表的状态
sysoListNode(l1);
sysoListNode(l2);
return l1;
} else {
l2.next = mergeTwoLists(l1, l2.next);
//此处时为了更直观看当前链表的状态
sysoListNode(l1);
sysoListNode(l2);
return l2;
}
} //测试数据
public static void main(String[] args) {
ListNode l1 = new ListNode(1);
ListNode l2 = new ListNode(2);
ListNode l3 = new ListNode(4);
l1.next = l2;
l2.next = l3; ListNode r1 = new ListNode(1);
ListNode r2 = new ListNode(3);
ListNode r3 = new ListNode(4);
r1.next = r2;
r2.next = r3; mergeTwoLists(l1, r1); }
26 break;
27 }
28
29 if (Integer.MAX_VALUE / 10 > total
30 || (Integer.MAX_VALUE / 10 == total && Integer.MAX_VALUE % 10 >= digit)) {
31 total = total * 10 + digit;
32 index++;
33 } else {
34 return sign > 0 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
35 }
36 }
37 return total * sign;
38
39 }
20. 有效的括号
1 class Solution {
2 public boolean isValid(String s) {
3 int n = s.length();
4 for (int i = 0; i < n / 2; i++) {
5 if (s.contains("{}"))
6 s = s.replace("{}", "");
7 if (s.contains("()"))
8 s = s.replace("()", "");
9 if (s.contains("[]"))
10 s = s.replace("[]", "");
11 }
12 if ("".equals(s)) {
13 return true;
14 }
15 return false;
16 }
17 }
26. Remove Duplicates from Sorted Array
1 public static int removeDuplicates(int[] nums) {
2 int count = 1;
3 for (int i = 1; i < nums.length; i++) {//用当前的数字和上一个被比较的数字进行比较，如果大于他就替换，本题默认一排序
4 if (nums[i] > nums[count - 1]) {
5 nums[count++] = nums[i];
6 }
7 }
8 return count;
9
10 }
27. Remove Element
1 public static int removeElement(int[] nums, int val) {
2 if (nums == null) {
3 return -1;
4 } else if (nums.length == 0) {
5 return 0;
6 } else {
7 int count = 0;//统计几个不相同，同时作为新数组的下标
8 for (int i = 0; i < nums.length; i++) {
9 if (nums[i] != val) {
10 nums[count++] = nums[i];//注意count++的执行顺序
11 }
12 }
13 return count;
14 }
15
16 }
41. First Missing Positive
1 public static int firstMissingPositive(int[] nums) {
2 if (nums.length == 0) {
3 return 1;
4 }
5
6 Set<Integer> numsSet = new HashSet<Integer>();
7 for (int i = 0; i < nums.length; i++) {
8 if (nums[i] < 1) {
9 continue;
10 } else {
11 numsSet.add(nums[i]);
12 }
13 }
14 List<Integer> numsList = new ArrayList<Integer>();
15 numsSet.forEach(n -> numsList.add(n));
16
17 // 1筛选过后的数组为空
18 if (numsList.size() == 0) {
19 return 1;
20 }
21
22 numsList.sort((a, b) -> a.compareTo(b.intValue()));
23
24 int index = 0;// 1当前数组下标
25 for (int i = 1;; i++) {
26 // 1预防数组越界
27 if (index < numsList.size() && numsList.get(index) == i) {
28 index++;
29 } else {
30 return i;
31 }
32 }
33
34 }
46. Permutations
/**
* 本题目标：对于给定数组列出所有的可能排列组合
* 实现方法：利用递归的思路
* 举个例子，当数组为【1，2，3】；先考虑第一个数为1时，后面的可能性，以此类推
* 注意：后面的可能性要以递归的思路去考虑，或者入栈出栈的思路。
* @param nums
* @return
*/
public static List<List<Integer>> permute(int[] nums) {
List<List<Integer>> list = new ArrayList<>();
// Arrays.sort(nums); // not necessary
backtrack(list, new ArrayList<>(), nums);
return list;
} private static void backtrack(List<List<Integer>> list, List<Integer> tempList, int[] nums) {
if (tempList.size() == nums.length) {
//注意这里的细节，是新声明一个集合去保存这个值，如果用tempList会导致最后list为空，原因就是堆被清空啦
list.add(new ArrayList<Integer>(tempList));
} else {
for (int i = 0; i < nums.length; i++) {
if (tempList.contains(nums[i])) {
continue;
} else {
tempList.add(nums[i]);
//注意是循环中调用递归
backtrack(list, tempList, nums);
//小算法，清空当前递归中的最后一个值
tempList.remove(tempList.size() - 1);
}
} }
}
48. Rotate Image
/**
* 本体题意：顺时针反转90度
* 解题方法：找出通项公式
* @param matrix
*/
public static void rotate(int[][] matrix) {
int n = matrix.length;
int[][] rotate = new int[n][n];
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
//通项公式
rotate[i][j] = matrix[n - 1 - j][i];
}
}
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
//重新赋值
matrix[i][j] = rotate[i][j];
}
}
}
49. Group Anagrams
/**
* 本题题意：将含有相同字母的字符串归类
*
* 解法：将字符串拆分成字符，然后排序作为key,最后map转化成list
*/
public static List<List<String>> groupAnagrams(String[] strs) {
if (strs.length == 0) {
return new ArrayList<List<String>>();
}
Map<String, List<String>> map = new HashMap<String, List<String>>();
for (String s : strs) {
char[] chars = s.toCharArray();
Arrays.sort(chars);
String key = String.valueOf(chars);
if (!map.containsKey(key)) {
map.put(key, new ArrayList<String>());
}
map.get(key).add(s);
}
return new ArrayList<List<String>>(map.values()); }
53. Maximum Subarray
/**
* 题意：找出一组 最大的数组和，作为结果的数组长度不限但小于等于给定的数组
* 解决方法：二步；
* 第一步找到当前最大（通过(当前最大的+a[i+1]) + a[i+1]比较，找出最大的）（类似贪心）
* 第二步找到当前最大的和之前最大的进行比较，选出最大的
* 注意：maxSum初始值一定要定义最小，可能为负数，如果初始化成0就不行啦
* @param A
* @return
*/
public static int maxSubArray(int[] A) {
if (A == null || A.length == 0) {
throw new IllegalArgumentException();
}
// ‘记录当前数据的最大值’；‘理解成新生成的最大值 和 旧的（已知的最大值比较）’
int maxSum = Integer.MIN_VALUE;
// ‘记录（当前）和（当前加下一位）的最大值’
int maxCurrentSum = 0;
for (int i = 0; i < A.length; i++) {
maxCurrentSum = Math.max(maxCurrentSum + A[i], A[i]);
maxSum = Math.max(maxCurrentSum, maxSum);
}
return maxSum; } /**
* 这个解决思路很好，也很好理解
* 解决思路：‘如果上一次结果为负数，则上一次结果置0，加下一次数’
* 也不需要考虑最小值的问题啦
* @param nums
* @return
*/
public static int maxSubArray2(int[] nums) {
int max = nums[0], tmp = max;
for (int i = 1; i < nums.length; i++) {
if (tmp < 0) tmp = 0;//‘精髓’
tmp += nums[i];
max = Math.max(tmp, max);
}
return max;
} public static void main(String[] args) {
int[] a = {-1,1};
System.out.println(maxSubArray2(a));
}
55. Jump Game
/**
* 题意：从a[0]开始跳转当前索引对应数值的步数，看能否跳到最后一步 解题方法：通过
* （i(当前索引)+nums[i](能跳转的最大长度)）和当前索引进行比较；如果可达到的位置小于当前位置；则可以判断不可到达
*
* @param nums
* @return
*/
public static boolean canJump(int[] nums) {
int reachable = 0;
for (int i = 0; i < nums.length; i++) {
if (i > reachable) {
return false;
}
// i(当前索引)+nums[i](能跳转的最大长度)
reachable = Math.max(reachable, i + nums[i]);
}
return true;
} public static void main(String[] args) {
int[] nums = { 3, 2, 1, 0, 4 };// false
// int[] nums = { 2, 3, 1, 1, 4 };//true
canJump(nums);
}
75. Sort Colors
/**
* 题目：‘将红白蓝归类排序；其实就是012归类排序’
* 解决方法：‘我这里用的冒泡排序，可以尝试一下别的排序方法’
* @param nums
*/
public static void sortColors(int[] nums) {
//‘注意下标’
for (int i = 0; i < nums.length - 1; i++) {
//‘注意下标’
for (int j = 0; j < nums.length - 1; j++) {
if (nums[j] > nums[j + 1]) {
int temp = nums[j];
nums[j] = nums[j + 1];
nums[j + 1] = temp;
}
}
}
} public static void main(String[] args) {
int[] nums = { 2, 0, 2, 1, 1, 0 };
sortColors(nums);
for (int i = 0; i < nums.length; i++) {
System.out.println(nums[i]);
}
}
78. Subsets
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List; /**
* Given a set of distinct integers, nums, return all possible subsets (the
* power set).
*
* Note: The solution set must not contain duplicate subsets.
*
* Example:
*
* Input: nums = [1,2,3] Output: [ [3], [1], [2], [1,2,3], [1,3], [2,3], [1,2],[]]
*
*给予一个不重复的整数集合，返回所有的可能的子集
*/
public class Lc78 { public static void main(String[] args) {
int[] nums = new int[3];
for (int i = 0; i < 3; i++) {
nums[i] = i + 1;
} for (List<Integer> lists : subsets2(nums)) {
for (Integer i : lists) {
System.out.print(i);
}
System.out.println();
}
} /**
* 利用深度优先搜索（dfs）
*/
private static List<List<Integer>> results = new ArrayList<>(); public static List<List<Integer>> subsets2(int[] nums) {
dfs(nums, 0, new LinkedList<>());
return results;
} private static void dfs(int[] nums, int start, LinkedList<Integer> list) {
results.add(new ArrayList<>(list));
for (int i = start; i < nums.length; i++) {
list.addLast(nums[i]);
dfs(nums, i + 1, list);
//遍历之后删除该节点避免重复
list.removeLast();
}
} }
80. Remove Duplicates from Sorted Array II
1 public static int removeDuplicates(int[] nums) {
2 int count = 2;
3 for (int i = 2; i < nums.length; i++) {
4 if (nums[i] > nums[count - 2]) {
5 nums[count++] = nums[i];
6 }
7 }
8 return count;
9 }
94. Binary Tree Inorder Traversal
import java.util.ArrayList;
import java.util.List;
import java.util.Stack; public class TreeNode {
int val;
TreeNode left;
TreeNode right; TreeNode(int x) {
val = x;
} /**
* 题目：‘有序遍历：给你一个二叉树，有序遍历他的节点；有序遍历意味着先便利左子树，之后依次倒叙遍历右子树；
* ’最好集合solution的动图，入栈出栈的形式更好理解
*
* ‘解决方法：利用入栈出栈的形式
*
* @param root
* @return
*/
public static List<Integer> inorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<Integer>();
//声明一个栈来存取节点
Stack<TreeNode> stack = new Stack<TreeNode>();
TreeNode curr = root;
//如果节点没有遍历完或者说栈不为空就继续一下流程
while (curr != null || !stack.isEmpty()) {
//如果当前节点不为空，就继续将当前节点压入栈
while (curr != null) {
stack.push(curr);
curr = curr.left;
}
//如果当前节点为空，意味着左子树遍历完了，那就出栈存值，然后遍历当前节点的右子树
curr = stack.pop();
res.add(curr.val);
curr = curr.right;
}
return res;
} public static void main(String[] args) {
// [1,null,2,3]
TreeNode treeNode = new TreeNode(1);
TreeNode treeNode1 = new TreeNode(2);
TreeNode treeNode2 = new TreeNode(3);
treeNode.right = treeNode1;
treeNode1.left = treeNode2;
inorderTraversal(treeNode);
} }
101. Symmetric Tree
public class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) {
val = x;
}
/**
* 题意：’判断给定的二叉树是不是对称的二叉树
* 解决方法：‘递归，理解简单。通过递归判断所有的节点是否对称；
* 思路：’将一个给定的二叉树‘复制一份’，从根节点开始，判断对应的节点（以根节点为对称轴）是否相同；
* @param t1
* @param t2
* @return
*/
public static boolean isMirro(TreeNode t1,TreeNode t2) {
if(t1 ==null && t2==null) {
return true;
}
if(t1 == null || t2 == null) {
return false;
}
//判断当前节点以及当前节点的左右树
return (t1.val == t2.val) && isMirro(t1.left, t2.right) && isMirro(t1.right, t2.left);
}
public static boolean isSymmetric(TreeNode root) {
return isMirro(root, root);
}
public static void main(String[] args) {
// [1,2,2,3,4,4,3]
TreeNode t1 = new TreeNode(1);
TreeNode t2 = new TreeNode(2);
TreeNode t3 = new TreeNode(2);
TreeNode t4 = new TreeNode(3);
TreeNode t5 = new TreeNode(4);
TreeNode t6 = new TreeNode(4);
TreeNode t7 = new TreeNode(3);
t1.left = t2;t1.right = t3;
t2.left = t4;t2.right = t5;
t3.left = t6;t3.right = t7;
System.out.println(isSymmetric(t1));
}
}
104. Maximum Depth of Binary Tree
public class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) {
val = x;
}
/**
* 题意：遍历二叉树得到最大路径的长度
* 解题：遍历可以用递归或者是栈；这里用的是递归，栈我没用明白，
* @param root
* @return
*/
public static int maxDepth(TreeNode root) {
if(root == null) {
return 0;
}
return 1+Math.max(maxDepth(root.left), maxDepth(root.right));
}
public static void main(String[] args) {
// [0,2,4,1,null,3,-1,5,1,null,6,null,8]
TreeNode t1 = new TreeNode(0);
TreeNode t2 = new TreeNode(2);
TreeNode t3 = new TreeNode(4);
TreeNode t4 = new TreeNode(1);
TreeNode t5 = new TreeNode(3);
TreeNode t6 = new TreeNode(-1);
TreeNode t7 = new TreeNode(5);
TreeNode t8 = new TreeNode(1);
TreeNode t9 = new TreeNode(6);
TreeNode t10 = new TreeNode(8);
t1.left = t2;
t1.right = t3;
t2.left = t4;
t4.left = t7;
t4.right = t8;
t3.left = t5;
t3.right = t6;
t5.left = t9;
t6.left = t10;
System.out.println(maxDepth(t1));
}
}
121. Best Time to Buy and Sell Stock
public class oneHundredone {
/**
* 题目：卖股票 小卖大卖
* 解题方法:动态找到当前最小的，然后一次判断当前的最大利润
* @param prices
* @return
*/
public static int maxProfit(int prices[]) {
//初始化，
int minPrice = Integer.MAX_VALUE;
int maxProfit = 0;
for (int i = 0; i < prices.length; i++) {
//找到最小的买入值
minPrice = Math.min(minPrice, prices[i]);
//找到最大的卖出值
maxProfit = Math.max(maxProfit, prices[i] - minPrice);
}
return maxProfit;
}
public static void main(String[] args) {
int[] stock = { 7, 6, 4, 3, 1 };
System.out.println(maxProfit(stock));
}
}
136. Single Number
1 public int singleNumber(int[] nums) {
2
3
4 Map<Integer, Integer> map = new HashMap<Integer, Integer>();
5
6 for (int i = 0; i < nums.length; i++) {
7 if (map.containsKey(nums[i])) {
8 map.remove(nums[i]);
9 } else {
10 map.put(nums[i], 1);
11 }
12
13 }
14
15 Integer result = 0;
16 for (Integer key : map.keySet()) {
17 result = key;
18 break;
19 }
20
21 return result;
22
23
24 }
137. Single Number II
1 public int singleNumber(int[] nums) {
2
3
4 Map<Integer, Boolean> map = new HashMap<Integer, Boolean>();
5
6 for (int i = 0; i < nums.length; i++) {
7 if (map.containsKey(nums[i])) {
8 map.put(nums[i], false);
9 } else {
10 map.put(nums[i], true);
11 }
12
13 }
14
15 List<Integer> result = new ArrayList<Integer>();
16 map.forEach((k, v) -> {
17 if (v) {
18 result.add(k);
19 }
20 });
21 return result.get(0);
22
23
24 }
139. Word Break
1 import java.util.ArrayList;
2 import java.util.List;
3
4 /**
5 * 题意：根据给定的字典判断字符串是否可以完全根据字典拆解 思路：利用dp(动态规划：把问题分解成原子级别，求解每个问题的最优解，最后汇聚就是问题的最优解)
6 *
7 *
8 */
9 public class WordBreak {
10 /**
11 * dp1 比较容易理解
12 *
13 * 遍历给定字符串的每一个字符，和字典机型比较，如果符合条件（wordDict.contains(sub) && (j == 0 || dp[j -
14 * 1])），就将该位置设置成true 若遍历所有之后，dp的最后一位是true，代表字符串按照字段拆解完全
15 *
16 * @param s
17 * @param wordDict
18 * @return
19 */
20 public static boolean wordBreak(String s, List<String> wordDict) {
21 if (s == null || "".equals(s)) {
22 return false;
23 }
24 int n = s.length();
25 boolean[] dp = new boolean[s.length()];
26 for (int i = 0; i < s.length(); i++) {
27 for (int j = 0; j <= i; j++) {
28 String sub = s.substring(j, i + 1);
29 if (wordDict.contains(sub) && (j == 0 || dp[j - 1])) {
30 dp[i] = true;
31 }
32 }
33 }
34 return dp[n - 1];
35 }
36
37 /**
38 * dp2
39 * 和dp1思路一样，只是优化了匹配过程，第二次直接遍历字段进行匹配，优化了3ms
40 * @param s
41 * @param wordDict
42 * @return
43 */
44 public static boolean wordBreak2(String s, List<String> wordDict) {
45 int n = s.length();
46 boolean[] dp = new boolean[n + 1];
47 dp[0] = true;
48 for (int i = 1; i <= s.length(); i++) {
49 for (String word : wordDict) {
50 int len = word.length();
51 if (i >= len && dp[i - len] && s.substring(i - len, i).equals(word)) {
52 dp[i] = true;
53 }
54 }
55 }
56 return dp[n];
57
58 }
59
60 /**
61 * dp3递归加dp，还没有完全理解，不过思路都很相似
62 * @param s
63 * @param wordDict
64 * @return
65 */
66 public static boolean wordBreak3(String s, List<String> wordDict) {
67 boolean[] memo = new boolean[s.length()];
68 return wordBreakHelper(s, wordDict, memo, 0);
69 }
70
71 public static boolean wordBreakHelper(String s, List<String> wordDict, boolean[] memo, int i) {
72 if (i >= s.length()) {
73 return true;
74 }
75 if (memo[i]) {
76 return false;
77 }
78 for (String word : wordDict) {
79 if (!s.startsWith(word, i)) {
80 continue;
81 }
82 boolean result = wordBreakHelper(s, wordDict, memo, i + word.length());
83 if (result) {
84 return true;
85 }
86 memo[i] = true;
87 }
88 return false;
89 }
90
91 public static void main(String[] args) {
92 String s = "leetcode";
93 List<String> dict = new ArrayList<String>();
94 dict.add("leet");
95 dict.add("code");
96 // System.out.println(wordBreak(s, dict));
97 System.out.println(wordBreak2(s, dict));
98 }
99 }
152. Maximum Product Subarray
/**
* Given an integer array nums, find the contiguous subarray within an array
* (containing at least one number) which has the largest product.
*
* 给一整数，求解最大的连续乘积，数组包含至少一个整数
*
*/
public class Lc152 {
/**
* 思路：dp
* 最优子结构，之前最大值乘当前值为最大值
* 边界值，初始值为nums[0]
*公式： 最大值 为 当前值 和 之前最大值乘以当前值 中的一个
* @param nums
* @return
*/
public static int maxProduct(int[] nums) {
if (nums.length == 1) {
return 0;
} int max = nums[0];
int dpMax = nums[0];
int dpMin = nums[0];
for (int i = 1; i < nums.length; i++) {
dpMax = Math.max(nums[i], Math.max(dpMax * nums[i], dpMin * nums[i]));
dpMin = Math.min(nums[i], Math.min(dpMax * nums[i], dpMin * nums[i]));
max = Math.max(max, dpMax);
} return max;
} public static void main(String[] args) {
int[] nums = { 2, 3, -2, 4 };
System.out.println(maxProduct(nums));
}
}
167. Two Sum II - Input array is sorted
1 public int[] twoSum(int[] numbers, int target) {
2
3 int[] position = new int[2];
4 for (int i = 0; i < numbers.length; i++) {
5 for (int j = i + 1; j < numbers.length; j++) {
6 if (numbers[i] + numbers[j] == target) {
7 position[0] = ++i;
8 position[1] = ++j;
9 break;
10 }
11 }
12 }
13
14 return position;
15
16 }
169. Majority Element
import java.util.Arrays;
public class majorityElement {
/**
* 题目： Given an array of size n, find the majority element. The majority element
* is the element that appears more than ⌊ n/2 ⌋ times.
*
* You may assume that the array is non-empty and the majority element always
* exist in the array.
*
* 解题： 利用测试用例数据
*
* @param nums
* @return
*/
public static int majorityElement(int[] nums) {
Arrays.sort(nums);
return nums[nums.length / 2];
}
/**
* 摩尔投票 是你就给你加一，不是你就减一，如果你是0就替换
*/
public static int majorityElement2(int[] nums) {
int res = nums[0];
int count = 1;
for (int i : nums) {
if (i == res) {
count++;
} else {
count--;
}
if (count == 0) {
res = i;
count++;
}
}
return res;
}
public static void main(String[] args) {
int a[] = { 2, 2, 1, 1, 1, 2, 2 };
// System.out.println(majorityElement(a));
System.out.println(majorityElement2(a));
}
}
198. House Robber
/**
* You are a professional robber planning to rob houses along a street. Each
* house has a certain amount of money stashed, the only constraint stopping you
* from robbing each of them is that adjacent houses have security system
* connected and it will automatically contact the police if two adjacent houses
* were broken into on the same night.
*
* Given a list of non-negative integers representing the amount of money of
* each house, determine the maximum amount of money you can rob tonight without
* alerting the police.
*
* @author 5109v12458
*
*/
public class HouseRobbe { /**
* 每次求得的now都是相隔的俩个数相加
* @param nums
* @return
*/
public static int rob(int[] nums) {
int last = 0;
int now = 0;
int temp = 0;
//an = (a(n-1)+n,an)
for (int i : nums) {
temp = now;
now = Math.max(last + i, now);
last = temp;
}
return now;
} public static void main(String[] args) {
int nums[] = { 1, 2, 4, 6 };
System.out.println(rob(nums));
}
}
204. 计数质数
1 class Solution {
2 public int countPrimes(int n) {
3 int[] num = new int[n];
4
5 for (int i = 0; i < n; i++) {
6 num[i] = 1;
7 }
8
9 for (int j = 2; j < n; j++) {
10 if (num[j] == 1) {
11 for (int k = 2; k * j < n; k++) {
12 num[j * k] = 0;
13 }
14 }
15 }
16 int sum = 0;
17 for (int i = 2; i < n; i++) {
18 if (num[i] == 1) {
19 sum++;
20 }
21 }
22 return sum;
23 }
24
25 }
206. Reverse Linked List
/**
* Reverse a singly linked list.
*
* Example:
*
* Input: 1->2->3->4->5->NULL Output: 5->4->3->2->1->NULL Follow up:
*
* A linked list can be reversed either iteratively or recursively. Could you
* implement both?
*
*/
public class ReverseLinkedList {
public static class ListNode {
int val;
ListNode next; ListNode(int x) {
val = x;
}
} /**
* 解法，通过另一条链表实现反转链表
* @param head
* @return
*/
public static ListNode reverseList(ListNode head) {
ListNode prev = null;
ListNode curr = head;
while (curr != null) {
ListNode temp = curr.next;
/*
* 目的是将当前节点的下一个几点作为prev的上一个节点，当前节点作为prev的当前节点
* 其他的步骤就是单纯的置换
*/
curr.next = prev;
prev = curr;
curr = temp;
}
return prev;
} public static void main(String[] args) {
ListNode node = new ListNode(0);
ListNode node1 = new ListNode(1);
ListNode node2 = new ListNode(2); node.next = node1;
node1.next = node2; reverseList(node);
}
}
226. Invert Binary Tree
Invert a binary tree.
Example:
Input:
4
/ \
2 7
/ \ / \
1 3 6 9
Output:
4
/ \
7 2
/ \ / \
9 6 3 1
/**
* Definition for a binary tree node. public class TreeNode { int val; TreeNode
* left; TreeNode right; TreeNode(int x) { val = x; } }
*/
class Lc226 {
public static class TreeNode {
int val;
TreeNode left;
TreeNode right; TreeNode(int x) {
val = x;
}
} /**
*
* 反转二叉树：先按照左子树反转，在右子树反转，在左右字数反转
*
* @return
*/
public TreeNode invertTree(TreeNode root) {
if (root == null) {
return null;
}
TreeNode left = invertTree(root.left);
TreeNode right = invertTree(root.right);
root.left = right;
root.right = left;
return root;
}
}
260. Single Number III
1 public int[] singleNumber(int[] nums) {
2
3 Map<Integer, Boolean> map = new HashMap<Integer, Boolean>();
4
5 for (int i = 0; i < nums.length; i++) {
6 if (map.containsKey(nums[i])) {
7 map.put(nums[i], false);
8 } else {
9 map.put(nums[i], true);
10 }
11
12 }
13
14 List<Integer> resultTemp = new ArrayList<Integer>();
15
16 map.forEach((k, v) -> {
17 if (v) {
18 resultTemp.add(k);
19 }
20 });
21
22 int[] result = new int[resultTemp.size()];
23
24 for (int i = 0; i < resultTemp.size(); i++) {
25 result[i] = resultTemp.get(i);
26 }
27 return result;
28
29 }
283. Move Zeroes
1 public static void moveZeroes(int[] nums) {
2 for (int i = 0; i < nums.length; i++) {
3 if (nums[i] != 0) {
4 continue;
5 } else {
6 int count = 0;
7 do {
8 swap(nums, i);//如果当前位置为0，则置换到最后一位
9 count++;
10 } while (nums[i] == 0 && count < nums.length - i);//如果当前位置是0，并且之前也没有0啦，则终止循环，终止条件有待优化
11 }
12 }
13 for (int i = 0; i < nums.length; i++) {
14 System.out.print(nums[i]);
15 }
16 }
17
18 public static void swap(int[] nums, int position) {//普通的置换算法，冒泡排序里的一段
19 for (int i = position; i < nums.length - 1; i++) {
20 int temp = nums[i];
21 nums[i] = nums[i + 1];
22 nums[i + 1] = temp;
23 }
24 }
412. Fizz Buzz
1 class Solution {
2 public List<String> fizzBuzz(int n) {
3 List<String> result = new ArrayList<>();
4 for (int i = 1; i <= n; i++) {
5 if (i % 3 == 0 && i % 5 == 0) {
6 result.add("FizzBuzz");
7 } else if (i % 3 == 0) {
8 result.add("Fizz");
9 } else if (i % 5 == 0) {
10 result.add("Buzz");
11 } else {
12 result.add("" + i);
13 }
14
15 }
16 return result;
17 }
18 }
448. Find All Numbers Disappeared in an Array
1 public List<Integer> findDisappearedNumbers(int[] nums) {
2
3
4 Map<Integer, Integer> numsMap = new HashMap<Integer, Integer>();
5 for (int i = 0; i < nums.length; i++) {
6 numsMap.put(nums[i], i);
7 }
8
9 // 1给定数组应该有的大小
10 int size = nums.length;
11
12 List<Integer> disappearedNumbers = new ArrayList<Integer>();
13 for (int i = 1; i <= size; i++) {
14 if (!numsMap.containsKey(i)) {
15 disappearedNumbers.add(i);
16 }
17 }
18
19 return disappearedNumbers;
20
21 }
442. Find All Duplicates in an Array
1 public List<Integer> findDuplicates(int[] nums) {
2
3 // 1对于给定数组进行排序
4 Map<Integer, Integer> numsMap = new HashMap<Integer, Integer>();
5 for (int i = 0; i < nums.length; i++) {
6 if(numsMap.containsKey(nums[i])) {
7 numsMap.put(nums[i], 2);
8 }else {
9 numsMap.put(nums[i], 1);
10 }
11 }
12
13 List<Integer> disappearedNumbers = new ArrayList<Integer>();
14 numsMap.forEach((k,v)->{
15 if(v==2) {
16 disappearedNumbers.add(k);
17 }
18 });
19
20 return disappearedNumbers;
21
22
23 }
461. Hamming Distance
/**
* Hamming Distance
*
* The Hamming distance between two integers is the number of positions at which
* the corresponding bits are different. Given two integers x and y, calculate
* the Hamming distance.
*
* 转换为2进制，有几个位置上的值不同，就叫做Hamming distance
*
*/
public class Lc461 {
public static void main(String[] args) {
System.out.println(hammingDistance(1, 4));
} public static int hammingDistance(int x, int y) {
int sum = x ^ y;
int res = 0;
res += sum % 2;
sum /= 2;
return res;
}
}
560. Subarray Sum Equals K
import java.util.HashMap;
import java.util.Map; /**
*找出最大连续子序列的个数
*
*/
public class Lc560 { /**
* 如果存在 sum-k=subArrayCount，則存在对应count的连续子序列
*
* @param nums
* @param k
* @return
*/
public static int subarraySum(int[] nums, int k) {
int count = 0;
int sum = 0;
Map<Integer, Integer> map = new HashMap<>();// map<sum,count(该sum-k出现的次数)>
map.put(0, 1);
for (int i = 0; i < nums.length; i++) {
sum += nums[i];
if (map.containsKey(sum - k)) {
count += map.get(sum - k);
}
map.put(sum, map.getOrDefault(sum, 0) + 1);
}
return count;
} public static void main(String[] args) {
int nums[] = { 3, 4, 7, 2, -3, 1, 4, 2 };
System.out.println(subarraySum(nums, 7));
}
}
581. Shortest Unsorted Continuous Subarray
import java.util.Arrays;
/*
* 581. Shortest Unsorted Continuous Subarray
* 题意：找出数组中需要排序的长度
* 难度：Easy
* 分类：Array
* 思路：
* Tips：可以考虑八大排序
*/
public class Lc581 {
public static int findUnsortedSubarray(int[] nums) {
int copyFromNums[] = new int[nums.length];
for (int i = 0; i < nums.length; i++) {
copyFromNums[i] = nums[i];
}
Arrays.sort(copyFromNums);
int startPosition = 0;
int endPosition = 0;
for (int i = 0; i < nums.length; i++) {
if (copyFromNums[i] != nums[i]) {
startPosition = i;
break;
}
} for (int i = 0; i < nums.length; i++) {
if (copyFromNums[i] != nums[i]) {
endPosition = i;
}
} int count = 0;
if (endPosition != startPosition) {
count = endPosition - startPosition + 1;
}
return count;
} public static void main(String[] args) {
int nums[] = { 2, 6, 4, 8, 10, 9, 15 };
System.out.println(findUnsortedSubarray(nums));
}
}
617. Merge Two Binary Trees
1 /**
2 * 题意：将俩个二叉树及进行合并，如果同一位置节点都存在，则合并，否则旧直接放上去
3 * 思路：相同位置进行合并，有则相加，无责这届放上去，注意用递归
4 * @param t1
5 * @param t2
6 * @return
7 */
8 public static TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
9 if (t1 == null && t2 == null) {
10 return null;
11 }
12 if (t1 == null) {
13 return t2;
14 }
15 if (t2 == null) {
16 return t1;
17 }
18
19 TreeNode t = new TreeNode(t1.val + t2.val);
20 t.left = mergeTrees(t1.left, t2.left);
21 t.right = mergeTrees(t1.right, t2.right);
22 return t;
23 }
771. Jewels and Stones
public class Lc771 {
public static int numJewelsInStones(String J, String S) {
String[] strJ = convertToAscall(J);
String[] strS = convertToAscall(S);
int count = 0;
for (int i = 0; i < strS.length; i++) {
for (int k = 0; k < strJ.length; k++) {
if (strS[i].equals(strJ[k])) {
count++;
break;
}
}
}
return count;
}
private static String[] convertToAscall(String s) {
StringBuffer sb = new StringBuffer();
char[] chars = s.toCharArray();
for (char c : chars) {
sb.append((int) c).append(",");
}
return sb.toString().split(",");
}
public static void main(String[] args) {
String S = "aAAbbbddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddb";
String J = "addddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddA";
System.out.println(numJewelsInStones(J, S));
}
}