Leetcode剑指Offer刷题指南：

Leetcode剑指Offer刷题-学习计划目录_DEGv587的博客-CSDN博客

剑指 Offer 21. 调整数组顺序使奇数位于偶数前面

解法：双指针交换（易理解版）

class Solution {
    public int[] exchange(int[] nums) {
        int start = 0, end = nums.length - 1;
        while (start < end - 1) {
            //start处为奇数时往后走
            if (nums[start] % 2 != 0) {
                start++;
            }
            //end处为偶数时往前走
            if (nums[end] % 2 == 0) {
                end--;
            }
            //start处为偶数，end处为奇数时，进行交换
            if (nums[start] % 2 == 0 && nums[end] % 2 != 0) {
                swap(nums, start, end);
            }
        }
        return nums;
    }

    private void swap(int[] nums, int start, int end) {
        int tmp = nums[start];
        nums[start] = nums[end];
        nums[end] = tmp;
    }
}

 优化版：

class Solution {
    public int[] exchange(int[] nums) {
        int start = 0, end = nums.length - 1;
        while (start <= end) {
            if (nums[start] % 2 != 0) {
                start++;
            } else if (nums[end] % 2 == 0) {
                end--;
            } else {
                swap(nums, start, end);
            }
        }
        return nums;
    }

    private void swap(int[] nums, int start, int end) {
        int tmp = nums[start];
        nums[start] = nums[end];
        nums[end] = tmp;
    }
}

简化版：

class Solution {
    public int[] exchange(int[] nums) {
        int start = 0, end = nums.length - 1;
        while (start <= end) {
            if (nums[start] % 2 == 1) {
                start++;
            } else if (nums[end] % 2 == 0) {
                end--;
            } else {
                int tmp = nums[start];
                nums[start] = nums[end];
                nums[end] = tmp;
            }
        }

        return nums;
    }
}

剑指 Offer 57. 和为s的两个数字

解法一：HashSet

class Solution {
    public int[] twoSum(int[] nums, int target) {
        HashSet<Integer> set = new HashSet<>();
        for (int x : nums) {
            if (!set.contains(target - x)) {
                set.add(x);
            } else {
                return new int[]{x, target - x};
            }
        }
        return null;
    }
}

解法二：二分法

class Solution {
    public int[] twoSum(int[] nums, int target) {
        for (int i = 0; i < nums.length; ++i) {
            int x = target - nums[i];
            int left = i, right = nums.length - 1;
            while (left <= right) {
                int mid = (left + right) >> 1;
                if (x < nums[mid]) {
                    right = mid - 1;
                } else if (x > nums[mid]) {
                    left = mid + 1;
                } else {
                    return new int[]{nums[i], nums[mid]};
                }
            }
        }
        return null;
    }
}

解法三：双指针

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int left = 0, right = nums.length - 1;
        while (left < right) {
            int sum = nums[left] + nums[right];
            if (sum < target) {
                left++;
            } else if (sum > target) {
                right--;
            } else {
                return new int[]{nums[left], nums[right]};
            }
        }
        return null;
    }
}

剑指 Offer 58 - I. 翻转单词顺序

解法一：分割 + 倒序

class Solution {
    public String reverseWords(String s) {
        String[] strs = s.trim().split(" ");
        StringBuilder sb = new StringBuilder();

        for (int i = strs.length - 1; i >= 0; --i) {
            if (strs[i].equals("")) {
                continue;
            }
            sb.append(strs[i].trim());
            if (i == 0) {
                break;
            } else {
                sb.append(" ");
            }
        }

        return sb.toString();
    }
}

解法二：双指针

class Solution {
    public String reverseWords(String s) {
        //去掉前后空格
        s = s.trim();
        StringBuilder sb = new StringBuilder();
        int left = s.length() - 1, right = left;
        while (left >= 0) {
            //找到第一个空格位置
            while (left >= 0 && s.charAt(left) != ' ') {
                left--;
            }
            //添加找到的第一个空格后的第一个单词
            sb.append(s.substring(left + 1, right + 1)).append(" ");
            //跳过中间空格，找到空格前的位置
            while (left >= 0 && s.charAt(left) == ' ') {
                left--;
            }
            right = left;
        }

        return sb.toString().trim();
    }
}