LeetCode算法题目解答汇总

本文转自《四火的唠叨》

只要不是特别忙或者特别不方便，最近一直保持着每天做几道算法题的规律，到后来随着难度的增加，每天做的题目越来越少。我的初衷就是练习，因为一方面我本身算法基础并不好，再一方面是因为工作以后传统意义上所谓算法的东西接触还是太少。为了题目查找方便起见，我把之前几篇陆陆续续贴出来的我对LeetCode上面算法题的解答汇总在下面，CTRL+F就可以比较方便地找到。由于LeetCode上的题在不断更新，因此我也会不定期地更新。下面表格里面的Acceptance和Difficulty的Easy/Medium/Hard的分类都是来自LeetCode上面的数据，而Difficulty括号里面的数值和Frequency则来自这个文档。另外，LeetCode数据库的那十道题（截止到目前）我记录在这里。

总体来说，做LeetCode的题目收获还是很大的。而这种可以很快获得代码正确性和性能即使的反馈的方式，对算法提高也是很有帮助的。有的题目困难，主要包括两方面的原因，有的是思路比较怪异，用常规思路去解题往往时间复杂度或者空间复杂度过高，还有的则是需要考虑的情况比较复杂，case众多，很容易遗漏。

Title	Acceptance	Difficulty	Frequency
Palindrome Number	28.8%	Easy (2)	2
ZigZag Conversion	23.4%	Easy (3)	1
Valid Sudoku	27.6%	Easy (2)	2
Add Binary	25.5%	Easy (2)	4
Valid Parentheses	28.2%	Easy (2)	5
Valid Palindrome	22.2%	Easy (2)	5
Balanced Binary Tree	32.5%	Easy (1)	2
Valid Number	11.0%	Easy (2)	5
Symmetric Tree	31.6%	Easy (1)	2
String to Integer (atoi)	14.2%	Easy (2)	5
Same Tree	41.8%	Easy (1)	1
Binary Tree Level Order Traversal	30.6%	Easy (3)	4
Binary Tree Level Order Traversal II	31.0%	Easy (3)	1
Roman to Integer	33.9%	Easy (2)	4
Reverse Integer	39.8%	Easy (2)	3
Remove Nth Node From End of List	29.3%	Easy (2)	3
Remove Element	33.0%	Easy (1)	4
Remove Duplicates from Sorted List	34.7%	Easy (1)	3
Climbing Stairs	34.0%	Easy (2)	5
Remove Duplicates from Sorted Array	32.2%	Easy (1)	3
Plus One	31.4%	Easy (1)	2
Path Sum	30.4%	Easy (1)	3
Pascal's Triangle II	30.1%	Easy (2)	1
Pascal's Triangle	31.1%	Easy (2)	1
Minimum Depth of Binary Tree	29.4%	Easy (1)	1
Merge Two Sorted Lists	33.2%	Easy (2)	5
Merge Sorted Array	31.8%	Easy (2)	5
Maximum Depth of Binary Tree	43.8%	Easy (1)	1
Longest Common Prefix	27.0%	Easy (2)	1
Count and Say	26.7%	Easy (2)	2
Length of Last Word	29.0%	Easy (1)	1
Implement strStr()	21.9%	Easy (4)	5
Divide Two Integers	16.6%	Medium (4)	3
3Sum	16.7%	Medium (3)	5
Evaluate Reverse Polish Notation	19.9%	Medium
Find Minimum in Rotated Sorted Array	31.7%	Medium
Word Search	19.8%	Medium (3)	4
Word Ladder	18.4%	Medium (3)	5
Flatten Binary Tree to Linked List	28.2%	Medium (3)	3
Gas Station	25.9%	Medium
Generate Parentheses	31.4%	Medium (3)	4
Gray Code	32.1%	Medium (4)	2
Word Break	21.3%	Medium
Validate Binary Search Tree	25.9%	Medium (3)	5
Insertion Sort List	25.3%	Medium
Integer to Roman	33.8%	Medium (3)	4
4Sum	21.4%	Medium (3)	2
Jump Game	27.2%	Medium (3)	2
Add Two Numbers	22.9%	Medium (3)	4
Anagrams	23.9%	Medium (3)	4
Decode Ways	16.2%	Medium (3)	4
Letter Combinations of a Phone Number	26.4%	Medium (3)	3
Linked List Cycle	35.7%	Medium
Linked List Cycle II	30.8%	Medium
Best Time to Buy and Sell Stock	31.2%	Medium (2)	1
Unique Paths II	27.9%	Medium (3)	3
Longest Palindromic Substring	20.6%	Medium (4)	2
Longest Substring Without Repeating Characters	22.2%	Medium (3)	2
Unique Paths	31.7%	Medium (2)	3
Unique Binary Search Trees II	27.3%	Medium (4)	1
Unique Binary Search Trees	36.5%	Medium (3)	1
Two Sum	18.4%	Medium (2)	5
Convert Sorted List to Binary Search Tree	27.3%	Medium (4)	3
Maximum Product Subarray	15.9%	Medium
Maximum Subarray	34.0%	Medium (3)	3
Triangle	26.6%	Medium (3)	1
Best Time to Buy and Sell Stock II	36.6%	Medium (3)	1
Swap Nodes in Pairs	32.4%	Medium (2)	4
Convert Sorted Array to Binary Search Tree	32.9%	Medium (2)	3
Container With Most Water	31.3%	Medium (3)	2
Minimum Path Sum	31.0%	Medium (3)	3
Surrounded Regions	14.2%	Medium (4)	3
Multiply Strings	20.5%	Medium (4)	3
Sum Root to Leaf Numbers	29.7%	Medium (2)	4
Subsets II	27.0%	Medium (4)	2
Next Permutation	25.4%	Medium (5)	2
3Sum Closest	27.0%	Medium (3)	1
Palindrome Partitioning	25.9%	Medium (3)	4
Subsets	27.9%	Medium (3)	4
Partition List	27.0%	Medium (3)	3
Construct Binary Tree from Inorder and Postorder Traversal	26.6%	Medium
Construct Binary Tree from Preorder and Inorder Traversal	26.5%	Medium
Combinations	30.0%	Medium (3)	4
Combination Sum II	24.7%	Medium (4)	2
Path Sum II	26.9%	Medium (2)	2
Permutation Sequence	22.3%	Medium (5)	1
Permutations	31.2%	Medium (3)	4
Sqrt(x)	22.3%	Medium (4)	4
Combination Sum	26.8%	Medium (3)	3
Populating Next Right Pointers in Each Node	35.3%	Medium (3)	3
Spiral Matrix II	30.8%	Medium (3)	2
Pow(x, n)	25.9%	Medium (3)	5
Spiral Matrix	20.6%	Medium (4)	2
Sort List	20.6%	Medium
Clone Graph	23.0%	Medium
Remove Duplicates from Sorted Array II	30.6%	Medium (2)	2
Sort Colors	32.1%	Medium (4)	2
Remove Duplicates from Sorted List II	24.8%	Medium (3)	3
Binary Tree Zigzag Level Order Traversal	26.5%	Medium (4)	3
Binary Tree Preorder Traversal	35.5%	Medium
Reorder List	20.4%	Medium
Restore IP Addresses	20.5%	Medium (3)	3
Single Number II	33.8%	Medium
Reverse Linked List II	26.1%	Medium (3)	2
Single Number	45.6%	Medium
Reverse Words in a String	14.0%	Medium
Simplify Path	19.9%	Medium (3)	1
Rotate Image	31.2%	Medium (4)	2
Rotate List	22.0%	Medium (3)	2
Binary Tree Inorder Traversal	35.5%	Medium (4)	3
Set Matrix Zeroes	30.8%	Medium (3)	5
Search a 2D Matrix	31.2%	Medium (3)	3
Search for a Range	27.4%	Medium (4)	3
Search Insert Position	34.9%	Medium (2)	2
Search in Rotated Sorted Array II	30.9%	Medium (5)	3
Text Justification	14.0%	Hard (4)	2
Search in Rotated Sorted Array	28.6%	Hard (4)	3
Binary Tree Maximum Path Sum	20.2%	Hard (4)	2
Reverse Nodes in k-Group	24.9%	Hard (4)	2
Binary Tree Postorder Traversal	31.0%	Hard
Candy	19.3%	Hard
Edit Distance	25.5%	Hard (4)	3
Recover Binary Search Tree	23.7%	Hard (4)	2
Populating Next Right Pointers in Each Node II	30.7%	Hard (4)	2
Permutations II	25.0%	Hard (4)	2
Best Time to Buy and Sell Stock III	22.4%	Hard (4)	1
Palindrome Partitioning II	18.3%	Hard (4)	3
N-Queens II	33.9%	Hard (4)	3
Substring with Concatenation of All Words	18.1%	Hard (3)	1
Sudoku Solver	20.9%	Hard (4)	2
N-Queens	25.9%	Hard (4)	3
Minimum Window Substring	18.1%	Hard (4)	2
Merge k Sorted Lists	21.2%	Hard (3)	4
Merge Intervals	20.9%	Hard (4)	5
Scramble String	22.8%	Hard (5)	2
Trapping Rain Water	28.9%	Hard (4)	2
Median of Two Sorted Arrays	17.6%	Hard (5)	3
Maximal Rectangle	21.5%	Hard (5)	1
Max Points on a Line	11.2%	Hard
LRU Cache	14.1%	Hard
Longest Valid Parentheses	19.7%	Hard (4)	1
Longest Consecutive Sequence	28.2%	Hard (4)	3
Copy List with Random Pointer	23.5%	Hard
Largest Rectangle in Histogram	21.5%	Hard (5)	2
Jump Game II	24.7%	Hard (4)	2
Interleaving String	19.5%	Hard (5)	2
Insert Interval	20.7%	Hard (4)	5
Wildcard Matching	14.3%	Hard (5)	3
Distinct Subsequences	25.0%	Hard (4)	2
Word Break II	16.6%	Hard
First Missing Positive	22.6%	Hard (5)	2
Word Ladder II	11.5%	Hard (1)	1
Find Minimum in Rotated Sorted Array II	27.9%	Hard
Regular Expression Matching	20.2%	Hard (5)	3

【2015-6-17】从上次贴出来我的解答以后，最新更新的题目，除了一些需要买书只能在电子书上面看解答的没法验证以外，凡是题目能够在线解答和验证答案的，我把我的全部和分析放在了下面。欢迎指正。

#	Title	Acceptance	Difficulty
226	Invert Binary Tree	37.6%	Easy
225	Implement Stack using Queues	30.0%	Medium
224	Basic Calculator	16.1%	Medium
223	Rectangle Area	26.0%	Easy
222	Count Complete Tree Nodes	19.8%	Medium
221	Maximal Square	20.6%	Medium
220	Contains Duplicate III	15.0%	Medium
219	Contains Duplicate II	26.2%	Easy
218	The Skyline Problem	17.0%	Hard
217	Contains Duplicate	35.9%	Easy
216	Combination Sum III	27.3%	Medium
215	Kth Largest Element in an Array	27.4%	Medium
214	Shortest Palindrome	16.3%	Hard
213	House Robber II	26.1%	Medium
212	Word Search II	15.0%	Hard
211	Add and Search Word – Data structure design	20.9%	Medium
210	Course Schedule II	19.1%	Medium
209	Minimum Size Subarray Sum	23.1%	Medium
208	Implement Trie (Prefix Tree)	25.0%	Medium
207	Course Schedule	21.2%	Medium
206	Reverse Linked List	32.0%	Easy
205	Isomorphic Strings	24.2%	Easy
204	Count Primes	18.9%	Easy
203	Remove Linked List Elements	26.0%	Easy
202	Happy Number	31.5%	Easy
201	Bitwise AND of Numbers Range	27.2%	Medium
200	Number of Islands	21.9%	Medium
199	Binary Tree Right Side View	26.9%	Medium
198	House Robber	28.8%	Easy
191	Number of 1 Bits	37.3%	Easy
190	Reverse Bits	28.3%	Easy
189	Rotate Array	17.8%	Easy
188	Best Time to Buy and Sell Stock IV	17.0%	Hard
187	Repeated DNA Sequences	19.2%	Medium
186	Reverse Words in a String II	31.1%	Medium
179	Largest Number	15.7%	Medium
174	Dungeon Game	17.5%	Hard
173	Binary Search Tree Iterator	29.2%	Medium
172	Factorial Trailing Zeroes	28.3%	Easy
171	Excel Sheet Column Number	36.6%	Easy
170	Two Sum III – Data structure design	24.7%	Easy
169	Majority Element	34.9%	Easy
168	Excel Sheet Column Title	18.1%	Easy
167	Two Sum II – Input array is sorted	43.3%	Medium
166	Fraction to Recurring Decimal	12.6%	Medium
165	Compare Version Numbers	15.1%	Easy
164	Maximum Gap	24.3%	Hard
163	Missing Ranges	24.0%	Medium
162	Find Peak Element	31.4%	Medium
161	One Edit Distance	24.3%	Medium
160	Intersection of Two Linked Lists	28.5%	Easy
159	Longest Substring with At Most Two Distinct Characters	30.3%	Hard
158	Read N Characters Given Read4 II – Call multiple times	22.2%	Hard
157	Read N Characters Given Read4	29.9%	Easy
156	Binary Tree Upside Down	34.4%	Medium
155	Min Stack	18.3%	Easy

【Updated：12/15/2015】227到310题

#	Title	Acceptance	Difficulty
310	Minimum Height Trees	24.0%	Medium
309	Best Time to Buy and Sell Stock with Cooldown	33.7%	Medium
307	Range Sum Query – Mutable	15.5%	Medium
306	Additive Number	23.2%	Medium
304	Range Sum Query 2D – Immutable	20.3%	Medium
303	Range Sum Query – Immutable	23.9%	Easy
301	Remove Invalid Parentheses	28.6%	Hard
300	Longest Increasing Subsequence	31.8%	Medium
299	Bulls and Cows	25.9%	Easy
297	Serialize and Deserialize Binary Tree	24.2%	Medium
295	Find Median from Data Stream	19.7%	Hard
292	Nim Game	50.0%	Easy
290	Word Pattern	27.0%	Easy
289	Game of Life	32.2%	Medium
287	Find the Duplicate Number	36.0%	Hard
284	Peeking Iterator	31.8%	Medium
283	Move Zeroes	42.3%	Easy
282	Expression Add Operators	21.5%	Hard
279	Perfect Squares	29.8%	Medium
278	First Bad Version	21.0%	Easy
275	H-Index II	31.7%	Medium
274	H-Index	27.1%	Medium
273	Integer to English Words	16.9%	Medium
268	Missing Number	37.5%	Medium
264	Ugly Number II	24.5%	Medium
263	Ugly Number	34.6%	Easy
260	Single Number III	40.7%	Medium
258	Add Digits	47.6%	Easy
257	Binary Tree Paths	24.9%	Easy
242	Valid Anagram	39.1%	Easy
241	Different Ways to Add Parentheses	30.6%	Medium
240	Search a 2D Matrix II	31.4%	Medium
239	Sliding Window Maximum	24.8%	Hard
238	Product of Array Except Self	39.5%	Medium
237	Delete Node in a Linked List	44.0%	Easy
236	Lowest Common Ancestor of a Binary Tree	27.7%	Medium
235	Lowest Common Ancestor of a Binary Search Tree	37.9%	Easy
234	Palindrome Linked List	25.3%	Easy
233	Number of Digit One	22.6%	Medium
232	Implement Queue using Stacks	33.8%	Easy
231	Power of Two	33.3%	Easy
230	Kth Smallest Element in a BST	34.0%	Medium
229	Majority Element II	24.2%	Medium
228	Summary Ranges	21.6%	Easy
227	Basic Calculator II	22.2%	Medium

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