LeetCode is a very good website to sharpen your programming/problem-solving skills. Of course, not all solutions here are optimal and your suggestions and contributions are welcomed.

#	Title	Solution	Difficulty
1	Two Sum	Java,Python
2	Add Two Numbers	Java,Python
3	Longest Substring Without Repeating Characters	Java,Python
7	Reverse Integer	Java
8	String to Integer (atoi)	Java,Python
9	Palindrome Number	Java
11	Container With Most Water	Java,Python
12	Integer to Roman	Java
13	Roman to Integer	Java
14	Longest Common Prefix	Java
15	3Sum	Java,Python
16	3Sum Closest	Java
17	Letter Combinations of a Phone Number	Java
19	Remove Nth Node From End of List	Java
20	Valid Parentheses	Java
21	Merge Two Sorted Lists	Java,Python
22	Generate Parentheses	Java
23	Merge k Sorted Lists	Python
24	Swap Nodes in Pairs	Java
25	Reverse Nodes in k-Group	Python
26	Remove Duplicates from Sorted Array	Java
27	Remove Element	Java
28	Implement strStr()	Java
29	Divide Two Integers	Python
31	Next Permutation	Python
32	Longest Valid Parentheses	Python
33	Search in Rotated Sorted Array	Java,Python
34	Find First and Last Position of Element in Sorted Array	Python
35	Search Insert Position	Java
36	Valid Sudoku	Java
38	Count and Say	Java
39	Combination Sum	Java,Python
40	Combination Sum II	Python
41	First Missing Positive	Python
42	Trapping Rain Water	Python
43	Multiply Strings	Java
46	Permutations	Java,Python
47	Permutations II	Python
48	Rotate Image	Python
49	Group Anagrams	Python
50	Pow(x, n)	Java
53	Maximum Subarray	Java,Python
54	Spiral Matrix	Java,Python
55	Jump Game	Java
56	Merge Intervals	Python
56	Insert Interval	Python
58	Length of Last Word	Java
59	Spiral Matrix II	Python
60	Permutation Sequence	Python
61	Rotate List	Java,Python
62	Unique Paths	Python
63	Unique Paths II	Python
64	Minimum Path Sum	Python
66	Plus One	Java
67	Add Binary	Java
69	Sqrt(x)	C++,Python
70	Climbing Stairs	Java
71	Simplify Path	Python
72	Edit Distance	Java,C++
73	Set Matrix Zeroes	Java
74	Search a 2D Matrix	Java
75	Sort Colors	Java,Python
76	Minimum Window Substring	Python
77	Combinations	Java,Python
78	Subsets	Python
79	Word Search	Python
80	Remove Duplicates from Sorted Array II	Java
81	Search in Rotated Sorted Array II	Python
82	Remove Duplicates from Sorted List II	C++,Java,Python
83	Remove Duplicates from Sorted List	Java
84	Largest Rectangle in Histogram	Python
85	Maximal Rectangle	Python
86	Partition List	Python
88	Merge Sorted Array	Java
92	Reverse Linked List II	Java
98	Validate Binary Search Tree	Java
100	Same Tree	Java
101	Symmetric Tree	Java
102	Binary Tree Level Order Traversal	Java,Python
103	Binary Tree Zigzag Level Order Traversal	Python
104	Maximum Depth of Binary Tree	Java,Python
105	Construct Binary Tree from Preorder and Inorder Traversal	Python
106	Construct Binary Tree from Inorder and Postorder Traversal	Python
107	Binary Tree Level Order Traversal II	Java
108	Convert Sorted Array to Binary Search Tree	Python
109	Convert Sorted List to Binary Search Tree	Python
110	Balanced Binary Tree	Java
111	Minimum Depth of Binary Tree	Java
112	Path Sum	Java,Python
113	Path Sum II	Java,Python
118	Pascal's Triangle	Java
119	Pascal's Triangle II	Java
121	Best Time to Buy and Sell Stock	Python
125	Valid Palindrome	Java
127	Word Ladder	Python,Java
129	Sum Root to Leaf Numbers	Java
131	Palindrome Partitioning	Python
134	Gas Station	Python
135	Candy	Python
136	Single Number	Java
137	Single Number II	Java
138	Copy List with Random Pointer	Java
139	Word Break	Java
141	Linked List Cycle	Java
142	Linked List Cycle II	Java
144	Binary Tree Preorder Traversal	Python
145	Binary Tree Postorder Traversal	Python
146	LRU Cache	Python
147	Insertion Sort List	Python
148	Sort List	Python
149	Max Points on a Line	Java
150	Evaluate Reverse Polish Notation	Python
151	Reverse Words in a String	Python
152	Maximum Product Subarray	Python
153	Find Minimum in Rotated Sorted Array	Python
155	Min Stack	Java,Python
160	Intersection of Two Linked Lists	Java,Python
162	Find Peak Element	Java,Python
165	Compare Version Numbers	Java,Python
166	Fraction to Recurring Decimal	Python
167	Two Sum II - Input array is sorted	Python
168	Excel Sheet Column Title	Python
169	Majority Element	Java,Python
171	Excel Sheet Column Number	Python
172	Factorial Trailing Zeroes	Python
173	Binary Search Tree Iterator	Python
175	Combine Two Tables	SQL
176	Second Highest Salary	SQL
177	Nth Highest Salary	SQL
178	Rank Scores	SQL
179	Largest Number	Python
180	Consecutive Numbers	SQL
181	Employees Earning More Than Their Managers	SQL
182	Duplicate Emails	SQL
183	Customers Who Never Order	SQL
184	Department Highest Salary	SQL
185	Department Top Three Salaries	SQL
187	Repeated DNA Sequences	Java,Python
188	Best Time to Buy and Sell Stock IV	Python
189	Rotate Array	Java,Python
190	Reverse Bits	Java,Python
191	Number of 1 Bits	Java,Python
192	Word Frequency	Shell
193	Valid Phone Numbers	Shell
194	Transpose File	Shell
195	Tenth Line	Shell
196	Delete Duplicate Emails	SQL
197	Rising Temperature	SQL
198	House Robber	Java,Python
199	Binary Tree Right Side View	Python
200	Number of Islands	Python
201	Bitwise AND of Numbers Range	Python
202	Happy Number	Java
203	Remove Linked List Elements	Java,Python
204	Count Primes	Python
205	Isomorphic Strings	Java,Python
206	Reverse Linked List	Java,Python
207	Course Schedule	Python
208	Implement Trie (Prefix Tree)	Python
209	Minimum Size Subarray Sum	Python
210	Course Schedule II	Python
211	Add and Search Word - Data structure design	Python
212	Word Search II	Python
213	House Robber II	Python
215	Kth Largest Element in an Array	Python
216	Combination Sum III	Python
217	Contains Duplicate	Java,Python
218	The Skyline Problem	Python
219	Contains Duplicate II	Java,Python
220	Contains Duplicate III	Python
221	Maximal Square	Python
222	Count Complete Tree Nodes	Python
226	Implement Stack using Queues	Python
227	Basic Calculator II	Python
228	Summary Ranges	Java,Python
229	Majority Element II	Java,Python
230	Kth Smallest Element in a BST	Python
231	Power of Two	Python
232	Implement Queue using Stacks	Python
233	Number of Digit One	C++,Python
234	Palindrome Linked List	Java,Python
235	Lowest Common Ancestor of a Binary Search Tree	Java,Python
236	Lowest Common Ancestor of a Binary Tree	Python
237	Delete Node in a Linked List	C++,Python
238	Product of Array Except Self	Python
242	Valid Anagram	Java
257	Binary Tree Paths	Java
258	Add Digits	C++
260	Single Number III	Java
262	Trips and Users	SQL
263	Ugly Number	Java
264	Ugly Number II	Java
268	Missing Number	Java
273	Integer to English Words	Java
274	H-Index	Java
275	H-Index II	Java
279	Perfect Squares	Java
283	Move Zeroes	C++
284	Peeking Iterator	Java
287	Find the Duplicate Number	Java,Python
290	Word Pattern	Java
292	Nim Game	C++
295	Find Median from Data Stream	Java
299	Bulls and Cows	Java
300	Longest Increasing Subsequence	Java
303	Range Sum Query - Immutable	Java
304	Range Sum Query 2D - Immutable	Java
317	Shortest Distance from All Buildings	Python
328	Odd Even Linked List	Java
330	Patching Array	Java
338	Counting Bits	Java
344	Reverse String	Java
367	Valid Perfect Square	C++
371	Sum of Two Integers	Java
389	Find the Difference	Python
392	Is Subsequence	Java,Python
394	Decode String	Python
437	Path Sum III	Java
438	Find All Anagrams in a String	Java
445	Add Two Numbers II	Python
448	Find All Numbers Disappeared in an Array	Java
461	Hamming Distance	Java
485	Max Consecutive Ones	Java
496	Next Greater Element I	Python
500	Keyboard Row	Python
503	Next Greater Element II	Java
532	K-diff Pairs in an Array	Python
538	Convert BST to Greater Tree	Java
543	Diameter of Binary Tree	Java,Python
560	Subarray Sum Equals K	Java
561	Array Partition I	Python
572	Subtree of Another Tree	Java
581	Shortest Unsorted Continuous Subarray	Java
595	Big Countries	SQL
596	Classes More Than 5 Students	SQL
601	Human Traffic of Stadium	SQL
605	Can Place Flowers	Java
617	Merge Two Binary Trees	Java
620	Not Boring Movies	SQL
626	Exchange Seats	SQL
627	Swap Salary	SQL
643	Maximum Average Subarray I	Python
645	Set Mismatch	Python
653	Two Sum IV - Input is a BST	Python
657	Judge Route Circle	Python
661	Image Smoother	Python
665	Non-decreasing Array	Python
669	Trim a Binary Search Tree	Python
670	Maximum Swap	Python
671	Second Minimum Node In a Binary Tree	Python
674	Longest Continuous Increasing Subsequence	Python
680	Valid Palindrome II	Python
682	Baseball Game	Python
686	Repeated String Match	Python
687	Longest Univalue Path	Python
690	Employee Importance	Python
693	Binary Number with Alternating Bits	Python
695	Max Area of Island	Python
696	Count Binary Substrings	Python
697	Degree of an Array	Python
700	Search in a Binary Search Tree	Python
704	Binary Search	Python
705	Design HashSet	Python
706	Design HashMap	Python
707	Design Linked List	Python
709	To Lower Case	Python
717	1-bit and 2-bit Characters	Python
720	Longest Word in Dictionary	Python
724	Find Pivot Index	Python
728	Self Dividing Numbers	Python
733	Flood Fill	Python
744	Find Smallest Letter Greater Than Target	Python
746	Min Cost Climbing Stairs	Python
747	Largest Number At Least Twice of Others	Python
762	Prime Number of Set Bits in Binary Representation	Python
766	Toeplitz Matrix	Python
771	Jewels and Stones	Python
784	Letter Case Permutation	Python
788	Rotated Digits	Python
796	Rotate String	Python
804	Unique Morse Code Words	Python
806	Number of Lines To Write String	Python
811	Subdomain Visit Count	Python
812	Largest Triangle Area	Python
819	Most Common Word	Python
821	Shortest Distance to a Character	Python
824	Goat Latin	Python
830	Positions of Large Groups	Python
832	Flipping an Image	Java
836	Rectangle Overlap	Python
840	Magic Squares In Grid	Python
844	Backspace String Compare	Python
849	Maximize Distance to Closest Person	Python
852	Peak Index in a Mountain Array	Python
859	Buddy Strings	Python
860	Lemonade Change	Python
867	Binary Gap	Python
868	Transpose Matrix	Python
872	Leaf-Similar Trees	Java
874	Walking Robot Simulation	Java
876	Middle of the Linked List	Python
880	Decoded String at Index	Python
881	Boats to Save People	Python
883	Projection Area of 3D Shapes	Java
884	Uncommon Words from Two Sentences	Python
885	Spiral Matrix III	Python
888	Fair Candy Swap	Python
892	Surface Area of 3D Shapes	Python
893	Groups of Special-Equivalent Strings	Python
894	All Possible Full Binary Trees	Python
933	Number of Recent Calls	Python
958	Check Completeness of a Binary Tree	Python
1029	Check If It Is a Straight Line	Python
1035	Uncrossed Lines	Python
1044	Longest Duplicate Substring	Python
1049	Last Stone Weight II	Python
1071	Greatest Common Divisor of Strings	Python
1143	Longest Common Subsequence	Python
1232	Check If It Is a Straight Line	Python
1277	Count Square Submatrices with All Ones	Python
1344	Angle Between Hands of a Clock	Python

• Contains a lot of interview questions and still constantly adding!
• Many languages are used for problems -- you can choose what you want!

Clone it. Have a look at the solutions or some particular question and get inpired!

You can choose any problem in LeetCode and bring your solution here. However, notice the folder structure of current repo

--algorithms         # contains solutions for algorithmic problems in LeetCode
  --3sum             # named after the question itself
    --Solution.java  # actual code for the solution. the problem description and maybe even explanation
                     #   for the solution are in the form of comments
--shell              # solutions for shell related problems
--databases          # solutions for database related problems

1. Fork the repository.
2. Solve the problem in LeetCode.
3. Create the solution file in the correct directory and add the link to README table.
4. Create pull request.
5. After review and discussions, the patch will be merged!

You are welcome to use Issue Trackers in the repository to bring up any questions!

MIT license. You are free to contribute or hack!