LeetCode

###LeetCode Algorithm

(Notes: "♥" means you need buy a book from Leetcode)

#	Title	Solution	Difficulty
223	Sliding Window Maximum	C++	Hard
222	Product of Array Except Self	C++	Medium
221	Delete Node in a Linked List	C++	Easy
220	Lowest Common Ancestor of a Binary Tree	C++	Medium
219	Lowest Common Ancestor of a Binary Search Tree	C++	Easy
218	Palindrome Linked List	C++	Easy
217	Number of Digit One	C++	Medium
216	Implement Queue using Stacks	C++	Easy
215	Power of Two	C++	Easy
214	Kth Smallest Element in a BST	C++	Medium
213	Majority Element II	C++	Medium
212	Summary Ranges	C++	Easy
211	Basic Calculator II	C++	Medium
210	Invert Binary Tree	C++	Easy
209	Implement Stack using Queues	C++	Medium
208	Basic Calculator	C++	Medium
207	Rectangle Area	C++	Easy
206	Count Complete Tree Nodes	C++	Medium
205	Maximal Square	C++	Medium
204	Contains Duplicate III	C++	Medium
203	Contains Duplicate II	C++	Easy
202	The Skyline Problem	C++	Hard
201	Contains Duplicate	C++	Easy
200	Combination Sum III	C++	Medium
199	Kth Largest Element in an Array	C++	Medium
198	Shortest Palindrome	C++	Hard
197	House Robber II	C++	Medium
196	Word Search II	C++	Hard
195	Add and Search Word - Data structure design	C++	Medium
194	Course Schedule II	C++	Medium
193	Minimum Size Subarray Sum	C++	Medium
192	Implement Trie (Prefix Tree)	C++	Medium
191	Course Schedule	C++	Medium
190	Reverse Linked List	C++	Easy
189	Isomorphic Strings	C++	Easy
188	Count Primes	C++	Easy
187	Remove Linked List Elements	C++	Easy
186	Happy Number	C++	Easy
185	Bitwise AND of Numbers Range	C++	Medium
184	Number of Islands	C++	Medium
183	Binary Tree Right Side View	C++	Medium
182	House Robber	C++	Easy
181	Number of 1 Bits	C++	Easy
180	Reverse Bits	C++	Easy
179	Rotate Array	C++	Easy
178	Best Time to Buy and Sell Stock IV	C++	Hard
177	Repeated DNA Sequences	C++	Medium
176	Reverse Words in a String II ♥	C++	Medium
175	Largest Number	C++	Medium
174	Dungeon Game	C++	Hard
173	Binary Search Tree Iterator	C++	Medium
172	Factorial Trailing Zeroes	C++	Easy
171	Excel Sheet Column Number	C++	Easy
170	Two Sum III - Data structure design ♥	C++	Easy
169	Majority Element	C++	Easy
168	Excel Sheet Column Title	C++	Easy
167	Two Sum II - Input array is sorted ♥	C++	Medium
166	Fraction to Recurring Decimal	C++	Medium
165	Compare Version Numbers	C++	Easy
164	Maximum Gap	C++	Hard
163	Missing Ranges ♥	C++	Medium
162	Find Peak Element	C++	Medium
161	One Edit Distance♥	C++	Medium
160	Intersection of Two Linked Lists	C++	Easy
159	Longest Substring with At Most Two Distinct Characters ♥	C++	Hard
158	Read N Characters Given Read4 II - Call multiple times ♥	C++	Hard
157	Read N Characters Given Read4 ♥	C++	Easy
156	Binary Tree Upside Down ♥	C++	Medium
155	Min Stack	C++	Easy
154	Find Minimum in Rotated Sorted Array II	C++	Hard
153	Find Minimum in Rotated Sorted Array	C++	Medium
152	Maximum Product Subarray	C++	Medium
151	Reverse Words in a String	C++	Medium
150	Evaluate Reverse Polish Notation	C++	Medium
149	Max Points on a Line	C++	Hard
148	Sort List	C++	Medium
147	Insertion Sort List	C++	Medium
146	LRU Cache	C++	Hard
145	Binary Tree Postorder Traversal	C++	Hard
144	Binary Tree Preorder Traversal	C++	Medium
143	Reorder List	C++	Medium
142	Linked List Cycle II	C++	Medium
141	Linked List Cycle	C++	Medium
140	Word Break II	C++	Hard
139	Word Break	C++	Medium
138	Copy List with Random Pointer	C++	Hard
137	Single Number II	C++	Medium
136	Single Number	C++	Medium
135	Candy	C++	Hard
134	Gas Station	C++	Medium
133	Clone Graph	C++	Medium
132	Palindrome Partitioning II	C++	Hard
131	Palindrome Partitioning	C++	Medium
130	Surrounded Regions	C++	Medium
129	Sum Root to Leaf Numbers	C++	Medium
128	Longest Consecutive Sequence	C++	Hard
127	Word Ladder II	C++	Hard
126	Word Ladder	C++	Medium
125	Valid Palindrome	C++	Easy
124	Binary Tree Maximum Path Sum	C++	Hard
123	Best Time to Buy and Sell Stock III	C++	Hard
122	Best Time to Buy and Sell Stock II	C++	Medium
121	Best Time to Buy and Sell Stock	C++	Medium
120	Triangle	C++	Medium
119	Pascal's Triangle II	C++	Easy
118	Pascal's Triangle	C++	Easy
117	Populating Next Right Pointers in Each Node II	C++	Hard
116	Populating Next Right Pointers in Each Node	C++	Medium
115	Distinct Subsequences	C++	Hard
114	Flatten Binary Tree to Linked List	C++	Medium
113	Path Sum II	C++	Medium
112	Path Sum	C++	Easy
111	Minimum Depth of Binary Tree	C++	Easy
110	Balanced Binary Tree	C++	Easy
109	Convert Sorted List to Binary Search Tree	C++	Medium
108	Convert Sorted Array to Binary Search Tree	C++	Medium
107	Binary Tree Level Order Traversal II	C++	Easy
106	Construct Binary Tree from Inorder and Postorder Traversal	C++	Medium
105	Construct Binary Tree from Preorder and Inorder Traversal	C++	Medium
104	Maximum Depth of Binary Tree	C++	Easy
103	Binary Tree Zigzag Level Order Traversal	C++	Medium
102	Binary Tree Level Order Traversal	C++	Easy
101	Symmetric Tree	C++	Easy
100	Same Tree	C++	Easy
99	Recover Binary Search Tree	C++	Hard
98	Validate Binary Search Tree	C++	Medium
97	Interleaving String	C++	Hard
96	Unique Binary Search Trees II	C++	Medium
95	Unique Binary Search Trees	C++	Medium
94	Binary Tree Inorder Traversal	C++	Medium
93	Restore IP Addresses	C++	Medium
92	Reverse Linked List II	C++	Medium
91	Subsets II	C++	Medium
90	Decode Ways	C++	Medium
89	Gray Code	C++	Medium
88	Merge Sorted Array	C++	Easy
87	Scramble String	C++	Hard
86	Partition List	C++	Medium
85	Maximal Rectangle	C++	Hard
84	Largest Rectangle in Histogram	C++	Hard
83	Remove Duplicates from Sorted List II	C++	Medium
82	Remove Duplicates from Sorted List	C++	Easy
81	Search in Rotated Sorted Array II	C++	Medium
80	Remove Duplicates from Sorted Array II	C++	Medium
79	Word Search	C++	Medium
78	Subsets	C++	Medium
77	Combinations	C++	Medium
76	Minimum Window Substring	C++	Hard
75	Sort Colors	C++	Medium
74	Search a 2D Matrix	C++	Medium
73	Set Matrix Zeroes	C++	Medium
72	Edit Distance	C++	Hard
71	Simplify Path	C++	Medium
70	Climbing Stairs	C++	Easy
69	Sqrt(x)	C++	Medium
68	Text Justification	C++	Hard
67	Plus One	C++	Easy
66	Valid Number	C++	Easy
65	Add Binary	C++	Easy
64	Merge Two Sorted Lists	C++	Easy
63	Minimum Path Sum	C++	Medium
62	Unique Paths II	C++	Medium
61	Unique Paths	C++	Medium
60	Rotate List	C++	Medium
59	Permutation Sequence	C++	Medium
58	Spiral Matrix II	C++	Medium
57	Length of Last Word	C++	Easy
56	Insert Interval	C++	Hard
55	Merge Intervals	C++	Hard
54	Jump Game	C++	Medium
53	Spiral Matrix	C++	Medium
52	Maximum Subarray	C++	Medium
51	N-Queens II	C++	Hard
50	N-Queens	C++	Hard
49	"Pow(x, n)"	C++	Medium
48	Anagrams	C++	Medium
47	Rotate Image	C++	Medium
46	Permutations II	C++	Hard
45	Permutations	C++	Medium
44	Jump Game II	C++	Hard
43	Wildcard Matching	C++	Hard
42	Multiply Strings	C++	Medium
41	Trapping Rain Water	C++	Hard
40	First Missing Positive	C++	Hard
39	Combination Sum II	C++	Medium
38	Combination Sum	C++	Medium
37	Count and Say	C++	Easy
36	Sudoku Solver	C++	Hard
35	Valid Sudoku	C++	Easy
34	Search Insert Position	C++	Medium
33	Search for a Range	C++	Medium
32	Search in Rotated Sorted Array	C++	Hard
31	Longest Valid Parentheses	C++	Hard
30	Next Permutation	C++	Medium
29	Substring with Concatenation of All Words	C++	Hard
28	Divide Two Integers	C++	Medium
27	Implement strStr()	C++	Easy
26	Remove Element	C++	Easy
25	Remove Duplicates from Sorted Array	C++	Easy
24	Reverse Nodes in k-Group	C++	Hard
23	Swap Nodes in Pairs	C++	Medium
22	Merge k Sorted Lists	C++	Hard
21	Generate Parentheses	C++	Medium
20	Valid Parentheses	C++	Easy
19	Remove Nth Node From End of List	C++	Easy
18	Letter Combinations of a Phone Number	C++	Medium
17	4Sum	C++	Medium
16	3Sum Closest	C++	Medium
15	3Sum	C++	Medium
14	Longest Common Prefix	C++	Easy
13	Roman to Integer	C++	Easy
12	Integer to Roman	C++	Medium
11	Container With Most Water	C++	Medium
10	Regular Expression Matching	C++	Hard
9	Palindrome Number	C++	Easy
8	String to Integer (atoi)	C++	Easy
7	Reverse Integer	C++	Easy
6	ZigZag Conversion	C++	Easy
5	Longest Palindromic Substring	C++	Medium
4	Add Two Numbers	C++	Medium
3	Longest Substring Without Repeating Characters	C++	Medium
2	Median of Two Sorted Arrays	C++	Hard
1	Two Sum	C++	Medium

###LeetCode Shell

#	Title	Solution	Difficulty
4	Tenth Line	Bash	Easy
3	Transpose File	Bash	Medium
2	Valid Phone Numbers	Bash	Easy
1	Word Frequency	Bash	Medium

###如何写无bug的程序是程序的晋级关键！ ###原作者针对如何写无bug的程序的建议：处理好程序的状态，避免case-by-case的的情况。

Leetcode这个网站上的题都是一些经典的公司用来面试应聘者的面试题，很多人通过刷这些题来应聘一些喜欢面试算法的公司，比如：Google、微软、Facebook、Amazon之类的这些公司，基本上是应试教育的功利主义。

我做这些题目的不是为了要去应聘这些公司，而是为了锻炼一下自己的算法和编程能力。因为我开始工作的时候基本没有这样的训练算法和编程的网站，除了大学里的“算法和数据结构”里的好些最基础最基础的知识，基本上没有什么训练。所以，当我看到有人在做这些题的时候，我也蠢蠢欲动地想去刷一下。

于是，我花了3-4个月的业余时间，我把Leetcode的154道题全部做完了。（这也是最近我没有太多的时间来写博客的原因，你可以看到我之前做的那个活动中有几个算法题来自于Leetcode）有人说我时间太多了，这里声明一下，我基本上都是利用了晚上10点以后的时间来做这些题的。

LeetCode的题大致分成两类：

1）基础算法的知识。这些题里面有大量的算法题，解这些题都是有套路的，不是用递归（深度优先DFS，广度优先BFS），就是要用动态规划（Dynamic Programming），或是拆半查找（Binary Search），或是回溯（Back tracing），或是分治法（Divide and Conquer），还有大量的对树，数组、链表、字符串和hash表的操作。通过做这些题能让你对这些最基础的算法的思路有非常扎实的了解和训练。对我而言，Dynamic Programming 是我的短板，尤其是一些比较复杂的问题，在推导递推公式上总是有思维的缺陷（数学是我的硬伤），通过做了这些题后，我能感到我在DP的思路上有了很大的收获。

2）编程题。比如：atoi，strstr，add two num，括号匹配，字符串乘法，通配符匹配，文件路径简化，Text Justification，反转单词等等，这些题的Edge Case, Corner Case有很多。这些题需要你想清楚了再干，只要你稍有疏忽，就会有几个case让你痛不欲生，而且一不小心就会让你的代码会写得又臭又长，无法阅读。通过做这些题，可以非常好的训练你对各种情况的考虑，以及你对程序代码组织的掌控（其实就是其中的状态变量）。还记得我在《函数式编程》中说的，程序中的状态是你程序变得复杂难维护的直接原因。

我觉得每个程序员都应该花时间和精力做这些题，因为你会从这些题中得到很大的收益。做完这些题后你一定会明白下面几个道理：

1）想清楚了再干。这个观点我以前就在《多些时间可以少些代码》说过。如果你拿到题就上去直接写代码的话，你一定会被各种case打回来了。然后呢，你一着急，你就会进入那种我在《开发团队的效率》中说的那种毫无效率case by case的开发模式，而你也进入了“平庸模式”。于是你就会出现下图那样的情况。 Case-by-Case Developement

Case-by-Case Development

2. 编程是脑力劳动，急不得。这个事情在这做这些题的时候你就会发现，要么是脑子转不过来了，要么就是明明就差一点了，但程序怎么都调不对。如果你越着急的话，你就会发现你会离目标越远，而花的时间也会更多。另外，你会发现这些题基本上都是50行代码内就可以搞定的，但是为了这50行以内的代码，你要花好多时间和精力。coding 50行代码在我们的日常工作中分分钟就完成，而Leetcode里的50行代码却没那么简单，也许，用这个你就可以区别什么是码农，什么是程序员了。

3）加班要不得。因为我总是在晚上10点以后做题，所以，基本上都是在加班状态中工作。这种状态过上两三天，你就会发现，整个大脑已经不转了，而且不但不转，还会犯很多低级错误，很多事情都想不清楚，一个晚上都在和程序的状态控制做搏斗，代码写得越来越乱，越来越没条理。于是这种时候，我都会休息几天，不做题了，然后再做题的时候，就觉得非常地清楚。可见加班 是编程最致命的敌人！

我把我的C++代码放到了Github上，大家也帮我review一下，看看有没有可以改善的。

https://github.com/haoel/leetcode

好了，不多说了，我希望大家有时间都去练练LeetCode，无论是找工作还是对你的编程能力会有非常大的提高。

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