LeetCode 算法题目解答汇总

• LeetCode 题目解答——Easy 部分
• LeetCode 题目解答——Medium 部分（上）
• LeetCode 题目解答——Medium 部分（下）
• LeetCode 题目解答——Hard 部分
• LeetCode 题目解答——155~226 题
• LeetCode 题目解答——第 227 到 310 题
• LeetCode 付费题目（一）
• LeetCode 题目解答—— 第 311 到 371 题
• LeetCode 题目解答—— 第 372 到 415 题

[Updated on 9/22/2017] 如今回头看来，里面很多做法都不是最佳的，有的从复杂度上根本就不是最优解，有的写的太啰嗦，有的则用了一些过于 tricky 的方法。我没有为了这个再更新，就让它们去吧。请读者知道这一点。

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

【Updated：1/19/2017】311 到 371 题

371	Sum of Two Integers	51.9%	Easy
368	Largest Divisible Subset	31.9%	Medium
367	Valid Perfect Square	36.9%	Medium
365	Water and Jug Problem	24.7%	Medium
363	Max Sum of Rectangle No Larger Than K	30.6%	Hard
357	Count Numbers with Unique Digits	44.2%	Medium
355	Design Twitter	23.5%	Medium
354	Russian Doll Envelopes	30.6%	Hard
352	Data Stream as Disjoint Intervals	38.2%	Hard
350	Intersection of Two Arrays II	42.6%	Easy
349	Intersection of Two Arrays	44.5%	Easy
347	Top K Frequent Elements	44.4%	Medium
345	Reverse Vowels of a String	36.6%	Easy
344	Reverse String	58.1%	Easy
343	Integer Break	43.8%	Medium
342	Power of Four	36.6%	Easy
341	Flatten Nested List Iterator	35.3%	Medium
338	Counting Bits	58.6%	Medium
337	House Robber III	40.2%	Medium
336	Palindrome Pairs	22.7%	Hard
335	Self Crossing	22.8%	Hard
334	Increasing Triplet Subsequence	36.7%	Medium
332	Reconstruct Itinerary	26.8%	Medium
331	Verify Preorder Serialization of a Binary Tree	33.9%	Medium
330	Patching Array	30.8%	Hard
329	Longest Increasing Path in a Matrix	34.0%	Hard
328	Odd Even Linked List	40.7%	Medium
327	Count of Range Sum	27.7%	Hard
326	Power of Three	38.6%	Easy
324	Wiggle Sort II	24.3%	Medium
322	Coin Change	25.8%	Medium
321	Create Maximum Number	22.9%	Hard
319	Bulb Switcher	41.5%	Medium
318	Maximum Product of Word Lengths	41.3%	Medium
316	Remove Duplicate Letters	27.5%	Hard
315	Count of Smaller Numbers After Self	32.5%	Hard
313	Super Ugly Number	36.5%	Medium
312	Burst Balloons	40.4%	Hard

[Updated 11/6/2017] LeetCode 300 题以内需要付费才能查看的所有题目解答。

156	Binary Tree Upside Down
157	Read N Characters Given Read4
158	Read N Characters Given Read4 II – Call multiple times
159	Longest Substring with At Most Two Distinct Characters
161	One Edit Distance
163	Missing Ranges
170	Two Sum III – Data structure design
186	Reverse Words in a String II
243	Shortest Word Distance
244	Shortest Word Distance II
245	Shortest Word Distance III
246	Strobogrammatic Number
247	Strobogrammatic Number II
248	Strobogrammatic Number III
249	Group Shifted Strings
250	Count Univalue Subtrees
251	Flatten 2D Vector
252	Meeting Rooms
253	Meeting Rooms II
254	Factor Combinations
255	Verify Preorder Sequence in Binary Search Tree
256	Paint House
259	3Sum Smaller
261	Graph Valid Tree
265	Paint House II
266	Palindrome Permutation
267	Palindrome Permutation II
269	Alien Dictionary
270	Closest Binary Search Tree Value
271	Encode and Decode Strings
272	Closest Binary Search Tree Value II
276	Paint Fence
277	Find the Celebrity
280	Wiggle Sort
281	Zigzag Iterator
285	Inorder Successor in BST
286	Walls and Gates
288	Unique Word Abbreviation
291	Word Pattern II
293	Flip Game
294	Flip Game II
296	Best Meeting Point
298	Binary Tree Longest Consecutive Sequence

[Updated on 3/25/2019] 372 到 415 题：

372	Super Pow	35.5%	Medium
373	Find K Pairs with Smallest Sums	33.3%	Medium
374	Guess Number Higher or Lower	38.9%	Easy
375	Guess Number Higher or Lower II	37.3%	Medium
376	Wiggle Subsequence	37.0%	Medium
377	Combination Sum IV	43.7%	Medium
378	Kth Smallest Element in a Sorted Matrix	48.6%	Medium
380	Insert Delete GetRandom O(1)	42.1%	Medium
381	Insert Delete GetRandom O(1) – Duplicates allowed	31.6%	Hard
382	Linked List Random Node	48.8%	Medium
383	Ransom Note	49.4%	Easy
384	Shuffle an Array	49.6%	Medium
385	Mini Parser	31.6%	Medium
386	Lexicographical Numbers	45.0%	Medium
387	First Unique Character in a String	49.4%	Easy
388	Longest Absolute File Path	38.9%	Medium
389	Find the Difference	52.8%	Easy
390	Elimination Game	43.3%	Medium
391	Perfect Rectangle	28.0%	Hard
392	Is Subsequence	46.3%	Medium
393	UTF-8 Validation	35.5%	Medium
394	Decode String	44.1%	Medium
395	Longest Substring with At Least K Repeating Characters	38.0%	Medium
396	Rotate Function	34.9%	Medium
397	Integer Replacement	31.1%	Medium
398	Random Pick Index	49.1%	Medium
399	Evaluate Division	46.9%	Medium
400	Nth Digit	30.2%	Easy
401	Binary Watch	45.1%	Easy
402	Remove K Digits	26.2%	Medium
403	Frog Jump	35.6%	Hard
404	Sum of Left Leaves	48.7%	Easy
405	Convert a Number to Hexadecimal	41.7%	Easy
406	Queue Reconstruction by Height	59.1%	Medium
407	Trapping Rain Water II	38.9%	Hard
409	Longest Palindrome	47.6%	Easy
410	Split Array Largest Sum	41.9%	Hard
412	Fizz Buzz	59.0%	Easy
413	Arithmetic Slices	55.4%	Medium
414	Third Maximum Number	28.7%	Easy
415	Add Strings	43.2%	Easy

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