LeetCode Record

🐼 A daily journey of honing my programming skills and documenting my insights solutions and std solutions.

I categorize problems based on their optimal solution algorithm. If there's no fixed algorithm, I label it as a trick (requiring creativity, no template algorithm). If a problem isn't listed in the summary table, it means it's simple enough to complete during my sophomore year.

• Scripts folder is used to store my accepted code. Feel free to copy it for submission, but I recommend reading my code and insights before writing your own solution.

  1. I use languages such as C, CPP, Python3, Java, and GO depending on my preference.

  2. Filenames with xxx_std indicate that I've used someone else's code, either because the question is easy, the implementation is straightforward (I'm feeling lazy), or the problem is too difficult. I often use grandyang's code !, and I'm grateful for his help!

• Insights folder contains records of my insights for each problem. If you're having trouble understanding the code, refer to the corresponding insight file (note that my solutions might not always be the best).

• Year_Month folder contains the daily challenge on LeetCode. I complete the weekday problems, and the Readme in this folder provides insights.

Valuable Problems

Hard to associate them with a fixed algorithm

• 91 Decode Ways
• 117 Populating Next Right Pointers in Each Node II
• 128 Longest Consecutive Sequence
• 141 Linked List Cycle
• 166 Fraction to Recurring Decimal
• 169 Majority Element
• 177 Nth Highest Salary
• 179 Largest Number
• 185 Department Top Three Salaries
• 192 Word Frequency
• 194 Transpose File
• 199 Binary Tree Right Side View
• 214 Shortest Palindrome
• 218 The Skyline Problem
• 233 Number of Digit One
• 239 Sliding Window Maximum
• 240 Search a 2D Matrix II
• 287 Find the Duplicate Number
• 420 Strong Password Checker
• 421 Maximum XOR of Two Numbers in an Array
• 456 132 Pattern
• 458 Poor Pigs
• 472 Concatenated Words
• 479 Largest Palindrome Product
• 502 IPO
• 516 Longest Palindromic Subsequence == LCS
• 517 Super Washing Machines
• 523 Continuous Subarray Sum
• 542 01 Matrix
• 546 Remove Boxes
• 591 Tag Validator
• 632 Smallest Range Covering Elements from K Lists
• 1044 Longest Duplicate Substring

Template

• Trie - 472
• PairHash
• Disjoint set
• Fenwick Tree
• Linked List Operation

Summary Table

#	Title	Solution	Insights	Difficulty
10	Regular Expression Matching	CPP	Recursion	Hard
11	Container With Most Water	CPP	Trcik	medium
12	Interger to Roman	CPP	Trcik	medium
15	3Sum	CPP	Divide and Conquer	medium
16	3Sum Closest	CPP	Divide and Conquer	medium
18	4Sum	CPP	Divide and Conquer	medium
22	Generate Parentheses	CPP	Recursion	medium
23	Merge k Sorted Lists	CPP	Merge	Hard
25	Reverse Nodes in k-Group	C++	Simulation	Hard
29	Divide Two Integers	C++	Bit Operation	Medium
30	Substring with Concatenation of All Words	C++	Data Structure	Hard
31	Next Permutation	PY	Trick	Medium
32	Longest Valid Parentheses	C++	DP	Hard
33	Search in Rotated Sorted Array	C++	Binary Search	Medium
34	Find First and Last Position of Element in Sorted Array	C++	Binary Search	Medium
36	Valid Sudoku	C++	Trick	Medium
37	Sudoku Solver	C++	DFS	Hard
40	Combination Sum II	C++	DP	Medium
41	First Missing Positive	C++	Bucket+Trick	Hard
42	Trapping Rain Water	C++	Data Structure	Hard
43	Multiply Strings	C++	Trick	Medium
44	Wildcard Matching	C++	Trick	Hard
45	Jump Game II	C++	DP	Hard
46	Permutations	C++	Trick	Medium
47	Permutations II	C++	Trick+Data Structure	Medium
48	Rotate Image	C++	Trick	Medium
49	Group Anagrams	C++	Data Structure	Medium
50	Pow(x, n)	C++	Quick Pow	Medium
51	N-Queens	C++	DFS	Hard
52	N-Queens II	C++	DFS	Hard
54	Spiral Matrix	C++	Trick	Medium
56	Merge Intervals	C++	Trick	Medium
57	Insert Interval	C++	Trick	Medium
58	Length of Last Word	C++	Trick	Easy
59	Spiral Matrix II	C++	Trick	Medium
60	Permutation Sequence	C++	DFS	Hard
61	Rotate List	PY	Trick	Medium
64	Minimum Path Sum	C++	DP	Medium
65	Valid Number	C++	STATE MACHINE	Hard
66	Plus One	C++	Arbitrary-precision Arithmetic	Easy
67	Add Binary	PY	Arbitrary-precision Arithmetic	Easy
68	Text Justification	C++	Trick	Hard
69	Sqrt(x)	C++	Trick	Easy
71	Simplify Path	C++	String Operations	Medium
72	Edit Distance	C++	DP	Hard
73	Set Matrix Zeroes	C++	Trick	Medium
74	Search a 2D Matrix	C++	Binary Search	Medium
75	Sort Colors	C++	Trick	Medium
76	Minimum Window Substring	C++	Greedy	Hard
77	Combinations	C++	DFS	Medium
79	Word Search	C++	DFS	Medium
80	Remove Duplicates from Sorted Array II	C++	Data Structure	Medium
81	Search in Rotated Sorted Array II	C++	Binary Search	Medium
82	Remove Duplicates from Sorted List II	C++	Linked List Operation	Medium
83	Remove Duplicates from Sorted List	C++	Linked List Operation	Easy
84	Largest Rectangle in Histogram	C++	Tricky	Hard
85	Maximal Rectangle	SQL	Tricky	Hard
86	Partition List	C++	Linked List Operation	Medium
87	Scramble String	C++	Recursion	Hard
88	Merge Sorted Array	C++	Merge Sort	Easy
89	Gray Code	C++	Bit Operation	Medium
90	Subsets II	C++	DFS	Medium
91	Decode Ways	C++	DP	Medium
92	Reverse Linked List II	C++	Linked List Operation	Medium
93	Restore IP Addresses	C++	Brute Force	Medium
94	Binary Tree Inorder Traversal	C++	DFS	Medium
95	Unique Binary Search Trees II	C++	DFS	Medium
97	Interleaving String	C++	DP	Hard
98	Validate Binary Search Tree	C++	DFS	Medium
99	Recover Binary Search Tree	C++	DFS	Hard
101	Symmetric Tree	C++	DFS	Easy
102	Binary Tree Level Order Traversal	C++	DFS	Medium
103	Binary Tree Zigzag Level Order Traversal	C++	DFS	Medium
104	Maximum Depth of Binary Tree	C++	DFS	Easy
105	Construct Binary Tree from Preorder and Inorder Traversal	C++	Recrusion	Medium
106	Construct Binary Tree from Inorder and Postorder Traversal	C++	Recursion	Medium
107	Binary Tree Level Order Traversal II	C++	DFS	Medium
108	Convert Sorted Array to Binary Search Tree	C++	Recrusion	Easy
109	Convert Sorted List to Binary Search Tree	C++	Recursion	Medium
110	Balanced Binary Tree	C++	DFS	Easy
111	Minimum Depth of Binary Tree	C++	DFS	Easy
112	Path Sum	C++	DFS	Easy
113	Path Sum II	C++	DFS	Medium
114	Flatten Binary Tree to Linked List	C++	Recursion with While	Medium
115	Distinct Subsequences	C++	DP	Hard
116	Populating Next Right Pointers in Each Node	C++	Recursion	Medium
117	Populating Next Right Pointers in Each Node II	C++	Recursion	Medium
118	Pascal's Triangle	C++	DP	Easy
119	Pascal's Triangle II	C++	DP	Easy
120	Triangle	C++	DP	Medium
123	Best Time to Buy and Sell Stock III	C++	DP	Hard
124	Binary Tree Maximum Path Sum	C++	DFS	Hard
126	Word Ladder II	C++	Memory Based Search	Hard
127	Word Ladder	C++	Memory Based Search	Hard
128	Longest Consecutive Sequence	C++	Brute Force with Data Structure	Hard
129	Sum Root to Leaf Numbers	C++	DFS	Medium
130	Surrounded Regions	C++	DFS	Medium
131	Palindrome Partitioning	C++	Brute Force	Medium
132	Palindrome Partitioning II	C++	DP	Hard
133	Clone Graph	C++	Brute Force	Medium
135	Candy	C++	DP	Hard
137	Single Number II	C++	BIT OPERATION	Medium
138	Copy List with Random Pointer	C++	Brute Force	Medium
139	Word Break	PY	DP	Medium
140	Word Break II	PY	DFS	Hard
141	Linked List Cycle	C++	Tricky	Easy
142	Linked List Cycle II	C++	Tricky	Medium
143	Reorder List	C++	Simulation	Medium
144	Binary Tree Preorder Traversal	C++	DFS	Medium
145	Binary Tree Postorder Traversal	C++	DFS	Medium
146	LRU Cache	C++	Data Structure	Medium
147	Insertion Sort List	C++	Simulation	Medium
149	Max Points on a Line	C++	Tricky	Hard
150	Evaluate Reverse Polish Notation	C++	Data Structure	Medium
151	Reverse Words in a String	PYs	Simulation	Medium
152	Maximum Product Subarray	C++	DP	Medium
153	Find Minimum in Rotated Sorted Array	C++	Binary Search	Medium
154	Find Minimum in Rotated Sorted Array II	C++	Binary Search	Hard
155	Min Stack	C++	Data Structure	Easy
160	Intersection of Two Linked Lists	C++	Linked List Operation	Easy
162	Find Peak Element	C++	Binary Search	Medium
164	Maximum Gap	C++	Tricky	Hard
165	Compare Version Numbers	C++	Data Structure	Medium
166	Fraction to Recurring Decimal	C++	Tricky	Medium
167	Two Sum II - Input array is sorted	C++	Binary Search	Easy
168	Excel Sheet Column Title	C++	Simulation	Easy
169	Majority Element	C++	Tricky	Easy
171	Excel Sheet Column Number	C++	Simulation	Easy
172	Factorial Trailing Zeroes	C++	Tricky	Easy
173	Binary Search Tree Iterator	C++	DFS	Medium
174	Dungeon Game	C++	DP	Hard
175	Combine Two Tables	SQL	SQL	Easy
176	Second Highest Salary	SQL	SQL	Easy
177	Nth Highest Salary	SQL	SQL	Medium
178	Rank Scores	SQL	SQL	Medium
179	Largest Number	C++	Recurssion	Medium
180	Consecutive Numbers	SQL	SQL	Medium
181	Employees Earning More Than Their Managers	SQL	SQL	Easy
182	Duplicate Emails	SQL	SQL	Easy
183	Customers Who Never Order	SQL	SQL	Easy
184	Department Highest Salary	SQL	SQL	Medium
185	Department Top Three Salaries	SQL	SQL	Hard
187	Repeated DNA Sequences	C++	Data Structure	Medium
188	Best Time to Buy and Sell Stock IV	C++	DP	Hard
189	Rotate Array	C++	Simulation	Medium
190	Reverse Bits	C++	Bit Operation	Easy
191	Number of 1 Bits	C++	Bit Operations	Easy
192	Word Frequency	SH	Linux	Medium
193	Valid Phone Numbers	SH	Linux	Easy
194	Transpose File	SH	Linux	Medium
195	Tenth Line	SH	Linux	Easy
196	Delete Duplicate Emails	SQL	SQL	Easy
197	Rising Temperature	SQL	SQL	Easy
199	Binary Tree Right Side View	C++	Tricky	Medium
200	Number of Islands	C++	DFS	Medium
201	Bitwise AND of Numbers Range	C++	Bit Operations	Medium
202	Happy Number	C++	Simulation	Easy
203	Remove Linked List Elements	C++	Linked List Operations	Easy
204	Count Primes	C++	Tricky	Easy
205	Isomorphic Strings	C++	Simulation	Easy
206	Reverse Linked List	C++	Linked List Operations	Easy
207	Course Schedule	C++	DFS	Medium
208	Implement Trie (Prefix Tree)	C++	Data Structure	Medium
209	Minimum Size Subarray Sum	C++	Tricky	Medium
210	Course Schedule II	C++	Brute Force	Medium
212	Word Search II	C++	Data Structure	Hard
214	Shortest Palindrome	C++	Recursion	Hard
215	Kth Largest Element in an Array	C++	Simulation	Medium
217	Contains Duplicate	C++	Data Structure	Easy
218	The Skyline Problem	C++	Simulation	Hard
219	Contains Duplicate II	C++	Data Structure	Easy
221	Maximal Square	C++	DP	Medium
222	Count Complete Tree Nodes	C++	Recursion	Medium
223	Rectangle Area	C++	Tricky	Medium
224	Basic Calculator	PY	Stack	Hard
225	Implement Stack using Queues	C++	Data Structure	Easy
226	Invert Binary Tree	C++	Recursion	Easy
227	Basic Calculator II	C++	Stack	Medium
230	Kth Smallest Element in a BST	C++	DFS	Medium
231	Power of Two	C++	Simulation	Easy
232	Implement Queue using Stacks	C++	Data Structure	Easy
233	Number of Digit One	C++	Tricky	Hard
234	Palindrome Linked List	C++	Linked List Operation	Easy
235	Lowest Common Ancestor of a Binary Search Tree	C++	Recursion	Easy
236	Lowest Common Ancestor of a Binary Tree	C++	Recursion	Medium
237	Delete Node in a Linked List	C++	Linked List Operation	Easy
238	Product of Array Except Self	C++	Tricky	Medium
239	Sliding Window Maximum	C++	Tricky	Hard
240	Search a 2D Matrix II	C++	Binary Search	Medium
241	Different Ways to Add Parentheses	C++	DP+Recursion	Medium
242	Valid Anagram	C++	Data Structure	Easy
257	Binary Tree Paths	C++	Recursion	Easy
258	Add Digits	C++	Simulation	Easy
260	Single Number III	C++	Bit Operations	Medium
262	Trips and Users	SQL	SQL	Hard
263	Ugly Number	C++	Simulation	Easy
264	Ugly Number II	C++	Simulation	Medium
268	Missing Number	C++	Tricky	Easy
273	Integer to English Words	C++	Simulation	Hard
274	H-Index	C++	Tricky	Medium
275	H-Index II	C++	Tricky	Medium
278	First Bad Version	C++	Binary Search	Easy
279	Perfect Squares	PY	DP	Medium
282	Expression Add Operators	C++	DFS	Hard
283	Move Zeroes	PY	Simulation	Easy
284	Peeking Iterator	C++	Simulation	Medium
287	Find the Duplicate Number	C++	Tricky	Medium
289	Game of Life	C++	Simulation	Medium
292	Nim Game	C++	Tricky	Easy
301	Remove Invalid Parentheses	C++	Brute Force	Hard
303	Range Sum Query - Immutable	C++	Data Structure	Easy
304	Range Sum Query 2D - Immutable	C++	Data Structure	Medium
306	Additive Number	C++	Brute Force	Medium
307	Range Sum Query - Mutable	C++	Data Structure	Medium
309	Best Time to Buy and Sell Stock with Cooldown	C++	DP	Medium
312	Burst Balloons	C++	DP	Hard
313	Super Ugly Number	C++	DP	Medium
315	Count of Smaller Numbers After Self	C++	Data Structure	Hard
318	Maximum Product of Word Lengths	C++	Brute Force	Medium
319	Bulb Switcher	C++	Trick	Medium
321	Create Maximum Number	C++	Greedy+DP	Hard
324	Wiggle Sort II	C++	Greedy+DP	Medium
327	Count of Range Sum	C++	Data Structure	Hard
328	Odd Even Linked List	C++	Data Structure	Medium
329	Longest Increasing Path in a Matrix	C++	DP+DFS	Hard
330	Patching Array	C++	Tricky	Hard
331	Verify Preorder Serialization of a Binary Tree	C++	Tricky	Medium
332	Reconstruct Itinerary	C++	DFS+Tricky	Medium
334	Increasing Triplet Subsequence	C++	Tricky	Medium
335	Self Crossing	C++	Tricky	Hard
336	Palindrome Pairs	C++	Data Structure	Hard
341	Flatten Nested List Iterator	C++	Data Structure	Medium
342	Power of Four	C++	Math	Easy
343	Integer Break	C++	Math	Medium
344	Reverse String	C++	Data Structure	Easy
345	Reverse Vowels of a String	C++	Simulation	Easy
347	Top K Frequent Elements	C++	Data Structure	Medium
349	Intersection of Two Arrays	C++	Data Structure	Easy
350	Intersection of Two Arrays II	C++	Data Structure	Easy
352	Data Stream as Disjoint Intervals	C++	Data Structure	Hard
355	Design Twitter	C++	Data Structure	Medium
357	Count Numbers with Unique Digits	C++	Trick	Medium
363	Max Sum of Rectangle No Larger Than K	C++	Data Structure	Hard
365	Water and Jug Problem	C++	Trick	Medium
367	Valid Perfect Square	C++	Trick	Easy
368	Largest Divisible Subset	C++	DP	Medium
371	Sum of Two Integers	C++	Trick	Medium
372	Super Pow	C++	Trick	Medium
373	Find K Pairs with Smallest Sums	C++	Trick	Medium
374	Guess Number Higher or Lower	C++	Binary Search	Easy
375	Guess Number Higher or Lower II	C++	Binary Search	Medium
378	Kth Smallest Element in a Sorted Matrix	C++	Binary Search	Medium
380	Insert Delete GetRandom O(1)	C++	Data Structure	Medium
381	Insert Delete GetRandom O(1) - Duplicates allowed	C++	Data Structure	Hard
383	Ransom Note	PY	String Operations	Easy
384	Shuffle an Array	C++	Data Structure	Medium
385	Mini Parser	C++	Recursion	Medium
386	Lexicographical Numbers	C++	Trick	Medium
387	First Unique Character in a String	C++	Data Structure	Easy
388	Longest Absolute File Path	C++	Data Structure	Medium
390	Elimination Game	C++	Data Structure	Medium
391	Perfect Rectangle	C++	Bit Operation	Hard
392	Is Subsequence	PY	String Operations	Easy
393	UTF-8 Validation	C++	Bit Operation	Medium
396	Rotate Function	C++	Tricky	Medium
397	Integer Replacement	C++	Tricky	Medium
398	Random Pick Index	C++	Tricky	Medium
400	Nth Digit	C++	Tricky	Medium
401	Binary Watch	C++	Bit Operation	Easy
402	Remove K Digits	C++	Data Structure	Easy
403	Frog Jump	C++	DP	Hard
404	Sum of Left Leaves	C++	Tree	Easy
405	Convert a Number to Hexadecimal	C++	Tree	Easy
406	Queue Reconstruction by Height	C++	Tree	Medium
407	Trapping Rain Water II	C++	BFS	Hard
409	Longest Palindrome	C++	Brute Force	Easy
410	Split Array Largest Sum	C++	Binary Search	Hard
412	Fizz Buzz	C++	Simulation	Easy
413	Arithmetic Slices	C++	Tricky	Medium
414	Third Maximum Number	C++	Simulation	Easy
416	Partition Equal Subset Sum	C++	Data Structure	Medium
417	Pacific Atlantic Water Flow	C++	Data Structure	Medium
419	Battleships in a Board	C++	DFS	Medium
420	Strong Password Checker	C++	Greedy	Hard
421	Maximum XOR of Two Numbers in an Array	C++	Data Structure	Medium
423	Reconstruct Original Digits from English	C++	Simulation	Medium
424	Longest Repeating Character Replacement	C++	Binary Search	Medium
427	Construct Quad Tree	C++	Recursion	Medium
429	N-ary Tree Level Order Traversal	C++	DFS	Medium
430	Flatten a Multilevel Doubly Linked List	C++	Recursion	Medium
432	All O`one Data Structure	C++	Data Structure	Hard
433	Minimum Genetic Mutation	C++	DFS	Medium
434	Number of Segments in a String	PY	String Operations	Easy
435	Non-overlapping Intervals	CPP	Greedy	Medium
436	Find Right Interval	CPP	Data Structure	Medium
437	Path Sum III	CPP	DFS	Medium
438	Find All Anagrams in a String	CPP	String Operations	Medium
440	K-th Smallest in Lexicographical Order	CPP	Tricky	Hard
441	Arranging Coins	CPP	Tricky	Easy
442	Find All Duplicates in an Array	CPP	Tricky	Medium
443	String Compression	CPP	String Operations	Medium
446	Arithmetic Slices II - Subsequence	CPP	DP / Tricky	Hard
447	Number of Boomerangs	CPP	Data Structure	Medium
448	Find All Numbers Disappeared in an Array	CPP	Tricky	Easy
450	Delete Node in a BST	CPP	Data Structure	Medium
451	Sort Characters By Frequency	PY	Data Structure	Medium
452	Minimum Number of Arrows to Burst Balloons	CPP	Tricky	Medium
453	Minimum Moves to Equal Array Elements	PY	Tricky	Medium
454	4Sum II	CPP	Data Structure	Medium
455	Assign Cookies	CPP	Greedy	Easy
456	132 Pattern	CPP	Stack	Medium
457	Circular Array Loop	CPP	Simulation	Medium
458	Poor Pigs	CPP	Mathematics	HARD
459	Repeated Substring Pattern	PY	String Operations	Easy
460	LFU Cache	CPP	Mathematics	Hard
461	Hamming Distance	CPP	Bit Operation	Easy
462	Minimum Moves to Equal Array Elements II	PY	Tricky	Medium
463	Island Perimeter	PY	Brute Force	Easy
464	Can I Win	PY	BFS	Medium
466	Count The Repetitions	CPP	Tricky+String Operations	Hard
467	Unique Substrings in Wraparound String	CPP	Tricky	Medium
468	Validate IP Address	PY	String Operations	Medium
470	Implement Rand10() Using Rand7()	CPP	Mathematics	Medium
472	Concatenated Words	CPP	Tricky+DP+String Operations	Hard
473	Matchsticks to Square	CPP	DP+Tricky+DFS	Medium
474	Ones and Zeroes	CPP	DP	Medium
475	Heaters	CPP	Binary Search	Medium
477	Total Hamming Distance	CPP	Bit Operations	Medium
478	Generate Random Point in a Circle	CPP	Mathematics	Medium
479	Largest Palindrome Product	CPP	Brute Force	Hard
480	Sliding Window Median	CPP	Data Structure	Hard
481	Magical String	CPP	Mathematics+String Operations	Medium
482	License Key Formatting	PY	String Operations	Easy
483	Smallest Good Base	CPP	Mathematics	Hard
485	Max Consecutive Ones	CPP	Tricky	Easy
486	Predict the Winner	CPP	DP	Medium
488	Zuma Game	PY	Brute Force	Hard
491	Increasing Subsequences	PY	Brute Force	Medium
492	Construct the Rectangle	CPP	Tricky	Easy
493	Reverse Pairs	CPP	Data Structure	Hard
494	Target Sum	CPP	Brute Force + DP	Medium
495	Teemo Attacking	CPP	Simulation	Easy
496	Next Greater Element I	CPP	Data Structure	Easy
497	Random Point in Non-overlapping Rectangles	CPP	Binary Search	Medium
498	Diagonal Traverse	PY	Tricky	Medium
500	Keyboard Row	PY	Tricky	Easy
501	Find Mode in Binary Search Tree	PY	Data Structure	Easy
502	IPO	PY	Greedy	Hard
503	Next Greater Element II	PY	Data Structure	Medium
504	Base 7	PY	Tricky	Easy
506	Relative Ranks	PY	Data Structure	Easy
507	Perfect Number	PY	Tricky	Easy
508	Most Frequent Subtree Sum	PY	Data Structure	Medium
509	Most Frequent Subtree Sum	PY	Simulation	Easy
513	Find Bottom Left Tree Value	PY	Data Structure	Medium
514	Freedom Trail	CPP	DP	Hard
515	Find Largest Value in Each Tree Row	PY	Data Structure	Medium
516	Longest Palindromic Subsequence	PY	DP	Medium
517	Super Washing Machines	PY	Greedy	Hard
518	Coin Change 2	CPP	DP	Medium
519	Random Flip Matrix	CPP	Tricky	Medium
520	Detect Capital	GO	Simulation	Easy
521	Longest Uncommon Subsequence I	GO	Simulation	Easy
522	Longest Uncommon Subsequence II	GO	Greedy	Medium
523	Continuous Subarray Sum	GO	Data Structure	Medium
524	Longest Word in Dictionary through Deleting	GO	String Operations	Medium
526	Beautiful Arrangement	GO	Brute Force	Medium
528	Random Pick with Weight	C++	Binary Search	Medium
529	Minesweeper	GO	Simulation	Medium
530	Minimum Absolute Difference in BST	GO	Simulation	Easy
532	K-diff Pairs in an Array	GO	Simulation	Medium
535	Encode and Decode TinyURL	GO	String Operation	Medium
538	Convert BST to Greater Tree	GO	DFS	Medium
539	Minimum Time Difference	GO	Tricky	Medium
540	Single Element in a Sorted Array	CPP	Binary Search	Medium
541	Reverse String II	GO	String Operation	Easy
542	01 Matrix	GO	DP	Medium
546	Remove Boxes	CPP	DP	Hard
547	Number of Provinces	GO	Brute Force	Medium
551	Student Attendance Record I	GO	Brute Force	Easy
552	Student Attendance Record II	C++	Mathematics	Hard
553	Optimal Division	GO	Tricky	Medium
554	Brick Wall	GO	Dynamic Programming	Medium
556	Next Greater Element III	GO	Data Structure	Medium
557	Reverse Words in a String III	PY	String Operations	Easy
558	Logical OR of Two Binary Grids Represented as Quad-Trees	PY	Recursion	Medium
559	Maximum Depth of N-ary Tree	PY	Recursion	Easy
560	Subarray Sum Equals K	PY	Data Structure	Medium
561	Array Partition	PY	Greedy	Easy
563	Binary Tree Tilt	PY	Recursion	Easy
564	Find the Closest Palindrome	PY	Tricky	Hard
566	Reshape the Matrix	PY	Tricky	Easy
567	Permutation in String	C++	Data Structure	Medium
572	Subtree of Another Tree	GO	Data Structure	Easy
575	Distribute Candies	GO	Simulation	Easy
576	Out of Boundary Paths	GO	Memory Based Search	Medium
581	Shortest Unsorted Continuous Subarray	GO	Tricky	Medium
583	Delete Operation for Two Strings	GO	Dynamic Programming	Medium
584	Find Customer Referee	SQL	Tricky	Easy
586	Customer Placing the Largest Number of Orders	SQL	Tricky	Easy
587	Erect the Fence	GO	Computing Geometry	Hard
590	N-ary Tree Postorder Traversal	GO	DFS	Easy
591	Tag Validator	PY	State Machine	Hard
592	Fraction Addition and Subtraction	PY	Mathematics	Medium
594	Longest Harmonious Subsequence	GO	Mathematics	Easy
595	Big Countries	SQL	Tricky	Easy
596	Classes More Than 5 Students	SQL	Trikcy	Easy
599	Minimum Index Sum of Two Lists	GO	Data Structure	Easy
600	Non-negative Integers without Consecutive Ones	GO	Mathematics	Hard
601	Human Traffic of Stadium	SQL	Tricky	Hard
602	Friend Requests II: Who Has the Most Friends	SQL	Tricky	Medium
605	Can Place Flowers	GO	Greedy	Easy
606	Construct String from Binary Tree	GO	String Operations	Easy
607	Sales Person	SQL	Tricky	Easy
608	Tree Node	SQL	Tricky	Easy
610	Triangle Judgement	SQL	Tricky	Easy
611	Valid Triangle Number	GO	Tricky	Medium
617	Merge Two Binary Trees	GO	Tricky	Easy
619	Biggest Single Number	SQL	Tricky	Easy
621	Task Scheduler	GO	Tricky	Medium
622	Design Circular Queue	GO	Tricky	Medium
623	Add One Row to Tree	GO	Tricky	Medium
626	Exchange Seats	SQL	Tricky	Medium
627	Swap Salary	SQL	Tricky	Easy
628	Maximum Product of Three Numbers	GO	Tricky	Easy
630	Course Schedule III	GO	Data Structure	Hard
632	Smallest Range Covering Elements from K Lists	GO	Tricky + Data Structure	Hard
636	Exclusive Time of Functions	GO	Tricky +Stack	Medium
637	Average of Levels in Binary Tree	GO	Data Structure	Easy
638	Shopping Offers	PY	DFS	Medium