Java Algorithm Problems

程序员的一天

从开始这个Github已经有将近两年时间, 很高兴这个repo可以帮到有需要的人. 我一直认为, 知识本身是无价的, 因此每逢闲暇, 我就会来维护这个repo, 给刷题的朋友们一些我的想法和见解. 下面来简单介绍一下这个repo:

README.md: 所有所做过的题目

ReviewPage.md: 所有题目的总结和归纳（不断完善中）

KnowledgeHash2.md: 对所做过的知识点的一些笔记

SystemDesign.md: 对系统设计的一些笔记

Future Milestone: 我准备将一些有意思的题目，做成视频的形式给大家参考

希望大家学习顺利, 对未来充满希望(程序员也是找到好老板的!) 有问题可以给我写邮件(wangdeve@gmail.com), 或者在GitHub上发issue给我.

Squence	Problem	Level	Language	Tags	Video Tutorial
0	Alien Dictionary.java	Hard	Java	[]
1	Anagrams.java	Medium	Java	[]
2	Binary Representation.java	Hard	Java	[]
3	Binary Tree Inorder Traversal.java	Easy	Java	[]
4	Binary Tree Level Order Traversal II.java	Medium	Java	[]
5	Binary Tree Level Order Traversal.java	Medium	Java	[]
6	Binary Tree Longest Consecutive Sequence.java	Medium	Java	[]
7	Binary Tree Maximum Path Sum II.java	Medium	Java	[]
8	Binary Tree Path Sum.java	Easy	Java	[]
9	Binary Tree Postorder Traversal.java	Easy	Java	[]
10	Binary Tree Preorder Traversal.java	Easy	Java	[]
11	Binary Tree Right Side View.java	Medium	Java	[]
12	Binary Tree Serialization.java	Medium	Java	[]
13	Binary Tree Zigzag Level Order Traversal.java	Medium	Java	[]
14	Building Outline.java	Hard	Java	[]
15	Change to Anagram.java	Easy	Java	[]
16	Classical Binary Search.java	Easy	Java	[]
17	Climbing Stairs.java	Easy	Java	[]
18	Closest Binary Search Tree Value.java	Easy	Java	[]
19	Closest Number in Sorted Array.java	Easy	Java	[]
20	ColorGrid.java	Medium	Java	[]
21	Combination Sum II.java	Medium	Java	[]
22	Combination Sum.java	Medium	Java	[]
23	Combinations.java	Medium	Java	[]
24	Compare Strings.java	Easy	Java	[]
25	Complete Binary Tree.java	Easy	Java	[]
26	Construct Binary Tree from Inorder and Postorder Traversal.java	Medium	Java	[]
27	Container With Most Water.java	Medium	Java	[]
28	Contains Duplicate II.java	Easy	Java	[]
29	Contains Duplicate III.java	Medium	Java	[]
30	Contains Duplicate.java	Easy	Java	[]
31	Convert Binary Search Tree to Doubly Linked List.java	Medium	Java	[]
32	Convert Expression to Polish Notation.java	Hard	Java	[]
33	Convert Expression to Reverse Polish Notation.java	Hard	Java	[]
34	Convert Integer A to Integer B.java	Easy	Java	[]
35	Copy List with Random Pointer.java	Medium	Java	[]
36	Cosine Similarity.java	Easy	Java	[]
37	Count 1 in Binary.java	Easy	Java	[]
38	Count and Say.java	Easy	Java	[]
39	Count of Smaller Number before itself.java	Hard	Java	[]
40	Count of Smaller Number.java	Medium	Java	[]
41	Count Primes.java	Easy	Java	[]
42	Course Schedule II.java	Medium	Java	[]
43	Course Schedule.java	Medium	Java	[]
44	Data Stream Median.java	Hard	Java	[]
45	Delete Digits.java	Medium	Java	[]
46	Delete Node in the Middle of Singly Linked List.java	Easy	Java	[]
47	Encode and Decode Strings.java	Medium	Java	[]
48	ExcelSheetColumnNumber .java	Easy	Java	[]
49	Expression Evaluation.java	Hard	Java	[]
50	Expression Tree Build.java	Hard	Java	[]
51	Fast Power.java	Medium	Java	[]
52	Fibonacci.java	Easy	Java	[]
53	Find Peak Element II.java	Review	Java	[]
54	Find the Connected Component in the Undirected Graph.java	Medium	Java	[]
55	Find the Weak Connected Component in the Directed Graph.java	Medium	Java	[]
56	First Missing Positive.java	N/A	Java	[]
57	Flatten 2D Vector.java	Medium	Java	[]
58	Flip Game II.java	Medium	Java	[]
59	Flip Game.java	Easy	Java	[]
60	Fraction to Recurring Decimal.java	Medium	Java	[]
61	Game of Life.java	N/A	Java	[]
62	Gas Station.java	N/A	Java	[]
63	Generate Parentheses.java	Medium	Java	[]
64	Gray Code.java	Medium	Java	[]
65	Group Anagrams.java	Medium	Java	[]
66	Group Shifted Strings.java	Easy	Java	[]
67	H-Index II.java	Medium	Java	[]
68	H-Index.java	Medium	Java	[]
69	Hamming Distance.java	Easy	Java	[]
70	Happy Number.java	Easy	Java	[]
71	Hash Function.java	Easy	Java	[]
72	HashHeap.java	Hard	Java	[]
73	HashWithArray.java	Easy	Java	[]
74	HashWithCustomizedClass(LinkedList).java	Medium	Java	[]
75	Heapify.java	Medium	Java	[]
76	Heaters.java	Easy	Java	[]
77	House Robber III.java	Hard	Java	[]
78	Identical Binary Tree.java	Easy	Java	[]
79	Implement Queue by Two Stacks.java	N/A	Java	[]
80	Implement Queue using Stacks.java	Easy	Java	[]
81	Implement Stack by Two Queues.java	Easy	Java	[]
82	Implement Stack using Queues.java	Easy	Java	[]
83	Implement Stack.java	Easy	Java	[]
84	Implement strStr().java	N/A	Java	[]
85	Implement Trie (Prefix Tree).java	Medium	Java	[]
86	IndexMatch.java	Easy	Java	[]
87	Inorder Successor in Binary Search Tree.java	Medium	Java	[]
88	Insert Interval.java	Easy	Java	[]
89	Insert Node in a Binary Search Tree .java	Easy	Java	[]
90	Insertion Sort List.java	N/A	Java	[]
91	Integer to English Words.java	N/A	Java	[]
92	Interleaving Positive and Negative Numbers.java	N/A	Java	[]
93	Intersection of Two Arrays.java	Easy	Java	[]
94	Intersection of Two Linked Lists.java	Easy	Java	[]
95	Interval Minimum Number.java	Medium	Java	[]
96	Interval Sum II.java	Hard	Java	[]
97	Interval Sum.java	Medium	Java	[]
98	Invert Binary Tree.java	Easy	Java	[]
99	Isomorphic Strings.java	Easy	Java	[]
100	Jump Game II.java	Hard	Java	[]
101	Kth Largest Element.java	Medium	Java	[]
102	Kth Smallest Element in a BST.java	N/A	Java	[]
103	Kth Smallest Sum In Two Sorted Arrays.java	Hard	Java	[]
104	Largest Number.java	N/A	Java	[]
105	Largest Rectangle in Histogram.java	N/A	Java	[]
106	Last Position of Target.java	N/A	Java	[]
107	Length of Last Word.java	N/A	Java	[]
108	Longest Common Prefix.java	Medium	Java	[]
109	Longest Common Substring.java	N/A	Java	[]
110	Longest Consecutive Sequence.java	N/A	Java	[]
111	Longest Increasing Continuous subsequence II.java	N/A	Java	[]
112	Longest Increasing Continuous subsequence.java	Easy	Java	[]
113	Longest Palindromic Substring.java	Medium	Java	[]
114	Longest Univalue Path.java	Easy	Java	[]
115	Longest Word in Dictionary.java	Easy	Java	[]
116	Longest Words.java	N/A	Java	[]
117	Lowest Common Ancestor II.java	Easy	Java	[]
118	Lowest Common Ancestor of a Binary Search Tree.java	Medium	Java	[]
119	Lowest Common Ancestor.java	Easy	Java	[]
120	LRU Cache.java	Hard	Java	[]
121	Majority Number II.java	Medium	Java	[]
122	Majority Number III.java	Medium	Java	[]
123	Matrix Zigzag Traversal.java	Easy	Java	[]
124	Max Area of Island.java	Easy	Java	[]
125	Max Tree.java	Hard	Java	[]
126	Maximal Square.java	Medium	Java	[]
127	Maximum Average Subarray II.java	Review	Java	[]
128	Maximum Depth of Binary Tree.java	Easy	Java	[]
129	Maximum Subarray III.java	N/A	Java	[]
130	Maximum Subarray.java	Easy	Java	[]
131	Maximum SubarrayII.java	N/A	Java	[]
132	MaximumSubarrayII.java	N/A	Java	[]
133	Median of two Sorted Arrays.java	Hard	Java	[]
134	Median.java	N/A	Java	[]
135	Meeting Rooms II.java	Medium	Java	[]
136	Meeting Rooms.java	Easy	Java	[]
137	Merge Intervals.java	Easy	Java	[]
138	Merge k Sorted Arrays.java	Medium	Java	[]
139	Merge k Sorted Lists.java	Medium	Java	[]
140	Merge Sorted Array II.java	N/A	Java	[]
141	Merge Sorted Array.java	Easy	Java	[]
142	Merge Two Binary Trees.java	Easy	Java	[]
143	Merge Two Sorted Lists.java	Easy	Java	[]
144	Middle of Linked List.java	N/A	Java	[]
145	Min Stack.java	Easy	Java	[]
146	Minimum Absolute Difference in BST.java	Easy	Java	[]
147	Minimum Height Trees.java	N/A	Java	[]
148	Minimum Path Sum.java	N/A	Java	[]
149	Minimum Subarray.java	N/A	Java	[]
150	MinimumDepthOfBinaryTree.java	Easy	Java	[]
151	Missing Ranges.java	N/A	Java	[]
152	Multiply Strings.java	Medium	Java	[]
153	Next Permutation.java	Medium	Java	[]
154	Nim Game.java	Easy	Java	[]
155	NQueens.java	N/A	Java	[]
156	NQueensII.java	N/A	Java	[]
157	Nth to Last Node in List.java	N/A	Java	[]
158	Number of Airplane in the sky.java	Medium	Java	[]
159	Number Triangles.java	N/A	Java	[]
160	O(1) Check Power of 2.java	N/A	Java	[]
161	One Edit Distance.java	Medium	Java	[]
162	Paint Fence.java	N/A	Java	[]
163	Palindrome Linked List.java	N/A	Java	[]
164	Palindrome Partitioning.java	N/A	Java	[]
165	Palindrome Permutation II.java	Medium	Java	[]
166	Palindrome Permutation.java	Easy	Java	[]
167	Partition Array by Odd and Even.java	N/A	Java	[]
168	Partition Array.java	N/A	Java	[]
169	Partition List.java	N/A	Java	[]
170	Pascal's Triangle II.java	Easy	Java	[]
171	Peeking Iterator.java	N/A	Java	[]
172	Permutation Index.java	Easy	Java	[]
173	Permutation Sequence.java	Medium	Java	[]
174	Permutations II.java	Medium	Java	[]
175	Permutations.java	Medium	Java	[]
176	Populating Next Right Pointers in Each Node II.java	Hard	Java	[]
177	Product of Array Exclude Itself.java	N/A	Java	[]
178	QuickSort.java	Easy	Java	[]
179	Recover Rotated Sorted Array.java	N/A	Java	[]
180	Rehashing.java	Medium	Java	[]
181	Remove Duplicates from Sorted Array.java	Easy	Java	[]
182	Remove Duplicates from Sorted List II.java	N/A	Java	[]
183	Remove Duplicates from Sorted List.java	Easy	Java	[]
184	Remove Duplicates from Unsorted List.java	N/A	Java	[]
185	Remove Invalid Parentheses.java	Hard	Java	[]
186	Remove Linked List Elements.java	N/A	Java	[]
187	Remove Node in Binary Search Tree.java	Hard	Java	[]
188	Reorder List.java	N/A	Java	[]
189	Reshape the Matrix.java	Easy	Java	[]
190	Restore IP Addresses.java	N/A	Java	[]
191	Reverse Integer.java	Easy	Java	[]
192	Reverse Linked List II .java	N/A	Java	[]
193	Reverse Linked List.java	Easy	Java	[]
194	Reverse String.java	Easy	Java	[]
195	Reverse Words in a String II.java	Medium	Java	[]
196	Reverse Words in a String.java	Medium	Java	[]
197	reverseInteger.java	Easy	Java	[]
198	Roman to Integer.java	Easy	Java	[]
199	Rotate Image.java	Medium	Java	[]
200	Rotate List.java	N/A	Java	[]
201	Rotate String.java	N/A	Java	[]
202	Russian Doll Envelopes.java	N/A	Java	[]
203	Search a 2D Matrix II.java	N/A	Java	[]
204	Search a 2D Matrix.java	N/A	Java	[]
205	Search for a Range.java	N/A	Java	[]
206	Search Insert Position.java	N/A	Java	[]
207	Search Range in Binary Search Tree .java	Medium	Java	[]
208	Search Rotated in Sorted Array II.java	N/A	Java	[]
209	Search Rotated in Sorted Array.java	Hard	Java	[]
210	Segment Tree Build II.java	Medium	Java	[]
211	Segment Tree Build.java	Medium	Java	[]
212	Segment Tree Modify.java	Medium	Java	[]
213	Segment Tree Query II.java	Medium	Java	[]
214	Segment Tree Query.java	Medium	Java	[]
215	Serilization and Deserialization Of Binary Tree.java	N/A	Java	[]
216	Shortest Word Distance.java	Easy	Java	[]
217	Single Number II.java	N/A	Java	[]
218	Single Number III.java	N/A	Java	[]
219	Single Number.java	Easy	Java	[]
220	Singleton.java	N/A	Java	[]
221	Sliding Window Maximum.java	N/A	Java	[]
222	Sliding Window Median.java	N/A	Java	[]
223	Sort Color.java	N/A	Java	[]
224	Sort Colors II.java	N/A	Java	[]
225	Sort Letters by Case.java	N/A	Java	[]
226	Sort List.java	N/A	Java	[]
227	Space Replacement.java	N/A	Java	[]
228	Sqrt(x).java	Easy	Java	[]
229	Stone Game.java	N/A	Java	[]
230	String Permutation.java	Easy	Java	[]
231	String to Integer(atoi).java	Easy	Java	[]
232	Strobogrammatic Number II.java	Medium	Java	[]
233	Strobogrammatic Number.java	Easy	Java	[]
234	StrStr.java	N/A	Java	[]
235	Subarray Sum Closest.java	Medium	Java	[]
236	Subarray Sum.java	Easy	Java	[]
237	Subset.java	Medium	Java	[]
238	Subsets II.java	Medium	Java	[]
239	Subtree.java	Easy	Java	[]
240	Summary Ranges.java	N/A	Java	[]
241	Swap Nodes in Pairs.java	N/A	Java	[]
242	Symmetric Binary Tree.java	Easy	Java	[]
243	The Smallest Difference.java	N/A	Java	[]
244	Top K Frequent Elements.java	Medium	Java	[]
245	Top K Frequent Words.java	Medium	Java	[]
246	Topological Sorting.java	Medium	Java	[]
247	Total Occurrence of Target.java	N/A	Java	[]
248	Trailing Zeros.java	N/A	Java	[]
249	Trapping Rain Water II.java	Hard	Java	[]
250	Tweaked Identical Binary Tree.java	Easy	Java	[]
251	Two Lists Sum.java	N/A	Java	[]
252	Two Strings Are Anagrams.java	Easy	Java	[]
253	Ugly Number II.java	N/A	Java	[]
254	Ugly Number.java	Medium	Java	[]
255	Unique Binary Search Tree II.java	N/A	Java	[]
256	Unique Characters.java	N/A	Java	[]
257	Unique Word Abbreviation.java	N/A	Java	[]
258	Valid Palindrome.java	Easy	Java	[]
259	Valid Parentheses.java	Easy	Java	[]
260	Valid Sudoku.java	Easy	Java	[]
261	Wood Cut.java	N/A	Java	[]
262	Word Break.java	Medium	Java	[]
263	Word Ladder II.java	Hard	Java	[]
264	Word Ladder.java	Medium	Java	[]
265	Word Pattern.java	Easy	Java	[]
266	Word Squares.java	Hard	Java	[]
267	Zigzag Iterator.java	N/A	Java	[]
268	Find Anagram Mappings.java	Easy	Java	[]
269	Judge Route Circle.java	Easy	Java	[]
270	Island Perimeter.java	Easy	Java	[]
271	First Unique Character in a String.java	Easy	Java	[]
272	Power of Three.java	Easy	Java	[]
273	Plus One.java	Easy	Java	[]
274	Power of Two.java	Easy	Java	[]
275	Reverse Vowels of a String.java	Easy	Java	[]
276	Guess Number Higher or Lower.java	Easy	Java	[]
277	Encode and Decode TinyURL.java	Medium	Java	[]
278	Wiggle Sort.java	Medium	Java	[]
279	Queue Reconstruction by Height.java	Medium	Java	[]
280	2 Sum.java	Easy	Java	[]	Link
281	2 Sum II - Input array is sorted.java	Medium	Java	[]
282	2 Sum II.java	Medium	Java	[]
283	Coin Change.java	Medium	Java	[]
284	Unique Path.java	Medium	Java	[]
285	Jump Game.java	Medium	Java	[]
286	Maximum Product Subarray.java	Medium	Java	[]
287	3 Sum Closest.java	Medium	Java	[]
288	Triangle Count.java	Medium	Java	[]
289	3 Sum.java	Medium	Java	[]
290	4 Sum.java	Medium	Java	[]
291	k Sum.java	Hard	Java	[]
292	Trapping Rain Water.java	Hard	Java	[Array, Stack, Two Pointers]
293	Longest Continuous Increasing Subsequence.java	Easy	Java	[]
294	Longest Increasing Subsequence.java	Medium	Java	[]
295	Unique Binary Search Tree.java	Medium	Java	[]
296	Trim a Binary Search Tree.java	Easy	Java	[]
297	Unique Paths II.java	Medium	Java	[]
298	Counting Bits.java	Medium	Java	[]
299	Bomb Enemy.java	Medium	Java	[]
300	Paint House.java	Easy	Java	[]
301	Decode Ways.java	Review	Java	[]
302	House Robber.java	Easy	Java	[]
303	House Robber II.java	Medium	Java	[]
304	Best Time to Buy and Sell Stock I.java	Easy	Java	[]
305	Best Time to Buy and Sell Stock II.java	Easy	Java	[]
306	Best Time to Buy and Sell Stock III .java	Hard	Java	[]
307	Best Time to Buy and Sell Stock IV.java	Hard	Java	[]
308	Paint House II.java	Review	Java	[]
309	3 Sum Smaller.java	Medium	Java	[]
310	Array Partition I.java	Easy	Java	[Array]
311	1-bit and 2-bit Characters.java	Easy	Java	[Array]
312	Non-decreasing Array.java	Easy	Java	[Array]
313	Max Consecutive Ones.java	Easy	Java	[Array]
314	Find All Numbers Disappeared in an Array.java	Easy	Java	[Array]
315	Maximum Average Subarray I.java	Easy	Java	[Array]
316	Largest Number At Least Twice of Others.java	Easy	Java	[Array]
317	Toeplitz Matrix.java	Easy	Java	[Array]
318	Backpack.java	Medium	Java	[DP]
319	Sum of Two Integers.java	Easy	Java	[Bit Manipulation]
320	Swap Bits.java	Easy	Java	[Bit Manipulation]
321	Update Bits.java	Medium	Java	[Bit Manipulation]
322	Maximum XOR of Two Numbers in an Array.java	Medium	Java	[Bit Manipulation, Trie]
323	Coins in a Line.java	Medium	Java	[DP, Greedy]
324	Perfect Squares.java	Medium	Java	[BFS, DP, Math]
325	Palindrome Partitioning II.java	Hard	Java	[DP]
326	Backpack V.java	Medium	Java	[DP]
327	Backpack VI.java	Medium	Java	[DP]
328	Copy Books.java	Review	Java	[DP]
329	Valid Perfect Square.java	Review	Java	[Binary Search, Math]
330	Intersection of Two Arrays II.java	Easy	Java	[Binary Search, Hash Table, Sort, Two Pointers]
331	First Bad Version.java	Easy	Java	[Binary Search]
332	Backpack II.java	Medium	Java	[DP]
333	Backpack III.java	Review	Java	[DP]
334	Longest Palindromic Subsequence.java	Medium	Java	[DP]
335	Burst Balloons.java	Hard	Java	[DP, Divide and Conquer]
336	Scramble String.java	Hard	Java	[DP, String]
337	Binary Search Tree Iterator.java	Medium	Java	[Design, Stack, Tree]
338	Flatten Nested List Iterator.java	Medium	Java	[Design, Stack]
339	Coins in a Line III.java	Hard	Java	[Array, DP]
340	Best Time to Buy and Sell Stock with Cooldown.java	Medium	Java	[DP]
341	Find Peak Element.java	Medium	Java	[Array, Binary Search]
342	Longest Common Subsequence.java	Medium	Java	[DP]
343	Interleaving String.java	Hard	Java	[DP, String]
344	Letter Combinations of a Phone Number.java	Medium	Java	[Backtracking, String]
345	Edit Distance.java	Hard	Java	[DP, String]
346	Distinct Subsequences.java	Hard	Java	[DP, String]
347	Regular Expression Matching.java	Review	Java	[Backtracking, DP, String]
348	Majority Element.java	Easy	Java	[Array, Bit Manipulation, Divide and Conquer]
349	Wildcard Matching.java	Hard	Java	[Backtracking, DP, Greedy, String]
350	Ones and Zeroes.java	Hard	Java	[DP]
351	Pow(x,n).java	Medium	Java	[Binary Search, Math]
352	Word Break II.java	Review	Java	[Backtracking, DP]
353	Nested List Weight Sum.java	Easy	Java	[DFS]
354	Same Tree.java	Easy	Java	[DFS, Tree]
355	Convert Sorted Array to Binary Search Tree.java	Easy	Java	[DFS, Tree]
356	Binary Tree Maximum Path Sum.java	Review	Java	[DFS, Tree]
357	Construct Binary Tree from Inorder and Preorder Traversal.java	Medium	Java	[Array, DFS, Tree]
358	Path Sum.java	Easy	Java	[DFS, Tree]
359	Add Binary.java	Easy	Java	[Math, String]
360	Add Digits.java	Easy	Java	[Math]
361	Add Two Numbers.java	Medium	Java	[Linked List, Math]
362	Add Two Numbers II.java	Medium	Java	[Linked List]
363	Balanced Binary Tree.java	Medium	Java	[DFS, Tree]
364	Valid Anagram.java	Easy	Java	[Hash Table, Sort]
365	Populating Next Right Pointers in Each Node.java	Medium	Java	[DFS, Tree]
366	Validate Binary Search Tree.java	Medium	Java	[DFS, Tree]
367	Convert Sorted List to Binary Search Tree.java	Medium	Java	[DFS, Linked List]
368	Flatten Binary Tree to Linked List.java	Medium	Java	[Binary Tree, DFS]
369	Binary Tree Paths.java	Easy	Java	[Binary Tree, DFS]
370	Minimum Size Subarray Sum.java	Medium	Java	[Array, Binary Search, Two Pointers]
371	Clone Graph.java	Medium	Java	[BFS, DFS, Graph]
372	Longest Substring Without Repeating Characters.java	Medium	Java	[Hash Table, String, Two Pointers]
373	Minimum Window Substring.java	Hard	Java	[Hash Table, String, Two Pointers]
374	Linked List Cycle.java	Easy	Java	[Linked List, Two Pointers]
375	Remove Nth Node From End of List.java	Medium	Java	[Linked List, Two Pointers]
376	Longest Substring with At Most K Distinct Characters.java	Medium	Java	[Hash Table, String]
377	Linked List Cycle II.java	Medium	Java	[Linked List, Two Pointers]
378	Kth Smallest Number in Sorted Matrix.java	Medium	Java	[Binary Search, Heap]
379	Find Minimum in Rotated Sorted Array.java	Medium	Java	[Array, Binary Search]
380	Find Minimum in Rotated Sorted Array II.java	Hard	Java	[Array, Binary Search]
381	Connecting Graph.java	Medium	Java	[Union Find]
382	Connecting Graph II.java	Medium	Java	[Union Find]
383	Connecting Graph III.java	Medium	Java	[Union Find]
384	Number of Islands.java	Medium	Java	[BFS, DFS, Union Find]
385	Number of Islands II.java	Hard	Java	[Union Find]
386	Graph Valid Tree.java	Medium	Java	[BFS, DFS, Graph, Union Find]
387	Surrounded Regions.java	Review	Java	[BFS, DFS, Union Find]
388	Implement Trie.java	Medium	Java	[Design, Trie]
389	Add and Search Word.java	Medium	Java	[Backtracking, Design, Trie]
390	Word Search II.java	Hard	Java	[Backtracking, DFS, Trie]
391	Word Search.java	Medium	Java	[Array, Backtracking]