Introduction of this Column

ID Title Difficulty Solution
1 Two Sum Easy C++
2 Add Two Numbers Medium no
3 Longest Substring Without Repeating Characters Medium no
5 Longest Palindromic Substring Medium no
6 ZigZag Conversion Easy no
7 Reverse Integer Easy no
8 String to Integer (atoi) Easy no
9 Palindrome Number Easy no
10 Regular Expression Matching Hard no
11 Container With Most Water Medium no
12 Integer to Roman Medium no
13 Roman to Integer Easy no
14 Longest Common Prefix Easy no
15 3Sum Medium no
16 3Sum Closest Medium no
17 Letter Combinations of a Phone Number Medium no
18 4Sum Medium no
19 Remove Nth Node From End of List Easy no
20 Valid Parentheses Easy no
21 Merge Two Sorted Lists Easy no
22 Generate Parentheses Medium no
23 Merge k Sorted Lists Hard no
24 Swap Nodes in Pairs Easy no
25 Reverse Nodes in k-Group Hard no
26 Remove Duplicates from Sorted Array Easy no
27 Remove Element Easy no
28 Implement strStr() Easy no
29 Divide Two Integers Medium no
31 Next Permutation Medium no
32 Longest Valid Parentheses Hard no
33 Search in Rotated Sorted Array Medium no
34 Search for a Range Medium no
35 Search Insert Position Medium no
36 Valid Sudoku Easy no
37 Sudoku Solver Hard no
38 Count and Say Easy no
39 Combination Sum Medium no
40 Combination Sum II Medium no
41 First Missing Positive Hard no
42 Trapping Rain Water Hard no
43 Multiply Strings Medium no
45 Jump Game II Hard no
46 Permutations Medium no
47 Permutations II Medium no
48 Rotate Image Medium no
49 Group Anagrams Medium no
50 Pow(x, n) Medium no
51 N-Queens Hard no
52 N-Queens II Hard no
53 Maximum Subarray Medium no
54 Spiral Matrix Medium no
55 Jump Game Medium no
56 Merge Intervals Medium no
57 Insert Interval Hard no
58 Length of Last Word Easy no
59 Spiral Matrix II Medium no
60 Permutation Sequence Medium no
61 Rotate List Medium no
62 Unique Paths Medium no
63 Unique Paths II Medium no
64 Minimum Path Sum Medium no
66 Plus One Easy no
67 Add Binary Easy no
68 Text Justification Hard no
69 Sqrt(x) Medium no
70 Climbing Stairs Easy no
71 Simplify Path Medium no
72 Edit Distance Hard no
73 Set Matrix Zeroes Medium no
74 Search a 2D Matrix Medium no
75 Sort Colors Medium no
77 Combinations Medium no
78 Subsets Medium no
79 Word Search Medium no
80 Remove Duplicates from Sorted Array II Medium no
81 Search in Rotated Sorted Array II Medium no
82 Remove Duplicates from Sorted List II Medium no
83 Remove Duplicates from Sorted List Easy no
86 Partition List Medium no
87 Scramble String Hard no
88 Merge Sorted Array Easy no
89 Gray Code Medium no
90 Subsets II Medium no
91 Decode Ways Medium no
92 Reverse Linked List II Medium no
93 Restore IP Addresses Medium no
94 Binary Tree Inorder Traversal Medium no
95 Unique Binary Search Trees II Medium no
96 Unique Binary Search Trees Medium no
97 Interleaving String Hard no
98 Validate Binary Search Tree Medium no
99 Recover Binary Search Tree Hard no
100 Same Tree Easy no
101 Symmetric Tree Easy no
102 Binary Tree Level Order Traversal Easy no
103 Binary Tree Zigzag Level Order Traversal Medium no
104 Maximum Depth of Binary Tree Easy no
105 Construct Binary Tree from Preorder and Inorder Traversal Medium no
106 Construct Binary Tree from Inorder and Postorder Traversal Medium no
107 Binary Tree Level Order Traversal II Easy no
108 Convert Sorted Array to Binary Search Tree Medium no
109 Convert Sorted List to Binary Search Tree Medium no
110 Balanced Binary Tree Easy no
111 Minimum Depth of Binary Tree Easy no
112 Path Sum Easy no
113 Path Sum II Medium no
114 Flatten Binary Tree to Linked List Medium no
116 Populating Next Right Pointers in Each Node Medium no
117 Populating Next Right Pointers in Each Node II Hard no
118 Pascal’s Triangle Easy no
119 Pascal’s Triangle II Easy no
120 Triangle Medium no
121 Best Time to Buy and Sell Stock Easy no
122 Best Time to Buy and Sell Stock II Medium no
123 Best Time to Buy and Sell Stock III Hard no
124 Binary Tree Maximum Path Sum Hard no
125 Valid Palindrome Easy no
127 Word Ladder Medium no
128 Longest Consecutive Sequence Hard no
129 Sum Root to Leaf Numbers Medium no
130 Surrounded Regions Medium no
131 Palindrome Partitioning Medium no
133 Clone Graph Medium no
134 Gas Station Medium no
135 Candy Hard no
136 Single Number Easy no
137 Single Number II Medium no
138 Copy List with Random Pointer Hard no
139 Word Break Medium no
140 Word Break II Hard no
141 Linked List Cycle Easy no
142 Linked List Cycle II Medium no
143 Reorder List Medium no
144 Binary Tree Preorder Traversal Medium no
145 Binary Tree Postorder Traversal Hard no
146 LRU Cache Hard no
147 Insertion Sort List Medium no
148 Sort List Medium no
149 Max Points on a Line Hard no
150 Evaluate Reverse Polish Notation Medium no
151 Reverse Words in a String Medium no
152 Maximum Product Subarray Medium no
153 Find Minimum in Rotated Sorted Array Medium no
154 Find Minimum in Rotated Sorted Array II Hard no
155 Min Stack Easy no
156 Binary Tree Upside Down Medium no
160 Intersection of Two Linked Lists Easy no
161 One Edit Distance Medium no
162 Find Peak Element Medium no
164 Maximum Gap Hard no
165 Compare Version Numbers Easy no
166 Fraction to Recurring Decimal Medium no
167 Two Sum II - Input array is sorted Medium no
168 Excel Sheet Column Title Easy no
169 Majority Element Easy no
170 Two Sum III - Data structure design Easy no
171 Excel Sheet Column Number Easy no
172 Factorial Trailing Zeroes Easy no
173 Binary Search Tree Iterator Medium no
179 Largest Number Medium no
186 Reverse Words in a String II Medium no
187 Repeated DNA Sequences Medium no
189 Rotate Array Easy no
190 Reverse Bits Easy no
191 Number of 1 Bits Easy no
198 House Robber Easy no
199 Binary Tree Right Side View Medium no
200 Number of Islands Medium no
201 Bitwise AND of Numbers Range Medium no
202 Happy Number Easy no
203 Remove Linked List Elements Easy no
204 Count Primes Easy no
205 Isomorphic Strings Easy no
206 Reverse Linked List Easy no
207 Course Schedule Medium no
208 Implement Trie (Prefix Tree) Medium no
209 Minimum Size Subarray Sum Medium no
212 Word Search II Hard no
213 House Robber II Medium no
215 Kth Largest Element in an Array Medium no
216 Combination Sum III Medium no
217 Contains Duplicate Easy no
219 Contains Duplicate II Easy no
220 Contains Duplicate III Medium no
221 Maximal Square Medium no
222 Count Complete Tree Nodes Medium no
223 Rectangle Area Easy no
225 Implement Stack using Queues Easy no
226 Invert Binary Tree Easy no
227 Basic Calculator II Medium no
228 Summary Ranges Medium no
229 Majority Element II Medium no
230 Kth Smallest Element in a BST Medium no
231 Power of Two Easy no
232 Implement Queue using Stacks Easy no
234 Palindrome Linked List Easy no
235 Lowest Common Ancestor of a Binary Search Tree Easy no
236 Lowest Common Ancestor of a Binary Tree Medium no
237 Delete Node in a Linked List Easy no
238 Product of Array Except Self Medium no
239 Sliding Window Maximum Hard no
240 Search a 2D Matrix II Medium no
241 Different Ways to Add Parentheses Medium no
242 Valid Anagram Easy no
243 Shortest Word Distance Easy no
244 Shortest Word Distance II Medium no
245 Shortest Word Distance III Medium no
246 Strobogrammatic Number Easy no
247 Strobogrammatic Number II Medium no
248 Strobogrammatic Number III Hard no
249 Group Shifted Strings Easy no
250 Count Univalue Subtrees Medium no
251 Flatten 2D Vector Medium no
252 Meeting Rooms Easy no
253 Meeting Rooms II Medium no
254 Factor Combinations Medium no
255 Verify Preorder Sequence in Binary Search Tree Medium no
256 Paint House Medium no
257 Binary Tree Paths Easy no
258 Add Digits Easy no
259 3Sum Smaller Medium no
260 Single Number III Medium no
261 Graph Valid Tree Medium no
263 Ugly Number Easy no
264 Ugly Number II Medium no
265 Paint House II Hard no
266 Palindrome Permutation Easy no
267 Palindrome Permutation II Medium no
268 Missing Number Medium no
269 Alien Dictionary Hard no
270 Closest Binary Search Tree Value Easy no
271 Encode and Decode Strings Medium no
272 Closest Binary Search Tree Value II Hard no
273 Integer to English Words Hard no
274 H-Index Medium no
275 H-Index II Medium no
276 Paint Fence Easy no
277 Find the Celebrity Medium no
278 First Bad Version Easy no
279 Perfect Squares Medium no
280 Wiggle Sort Medium no
281 Zigzag Iterator Medium no
282 Expression Add Operators Hard no
283 Move Zeroes Easy no
284 Peeking Iterator Medium no
285 Inorder Successor in BST Medium no
286 Walls and Gates Medium no
287 Find the Duplicate Number Hard no
288 Unique Word Abbreviation Easy no
289 Game of Life Medium no
290 Word Pattern Easy no
291 Word Pattern II Hard no
292 Nim Game Easy no
293 Flip Game Easy no
294 Flip Game II Medium no
295 Find Median from Data Stream Hard no
296 Best Meeting Point Hard no
297 Serialize and Deserialize Binary Tree Hard no
298 Binary Tree Longest Consecutive Sequence Medium no
299 Bulls and Cows Easy no
300 Longest Increasing Subsequence Medium no
301 Remove Invalid Parentheses Hard no
302 Smallest Rectangle Enclosing Black Pixels Hard no
303 Range Sum Query - Immutable Easy no
304 Range Sum Query 2D - Immutable Medium no
305 Number of Islands II Hard no
306 Additive Number Medium no
307 Range Sum Query - Mutable Medium no
308 Range Sum Query 2D - Mutable Hard no
309 Best Time to Buy and Sell Stock with Cooldown Medium no
310 Minimum Height Trees Medium no
311 Sparse Matrix Multiplication Medium no
312 Burst Balloons Hard no
313 Super Ugly Number Medium no
314 Binary Tree Vertical Order Traversal Medium no
315 Count of Smaller Numbers After Self Hard no
316 Remove Duplicate Letters Hard no
317 Shortest Distance from All Buildings Hard no
318 Maximum Product of Word Lengths Medium no
319 Bulb Switcher Medium no
320 Generalized Abbreviation Medium no
321 Create Maximum Number Hard no
322 Coin Change Medium no
323 Number of Connected Components in an Undirected Graph Medium no
324 Wiggle Sort II Medium no
325 Maximum Size Subarray Sum Equals k Medium no
326 Power of Three Easy no
327 Count of Range Sum Hard no
328 Odd Even Linked List Medium no
329 Longest Increasing Path in a Matrix Hard no
330 Patching Array Hard no
331 Verify Preorder Serialization of a Binary Tree Medium no
332 Reconstruct Itinerary Medium no
333 Largest BST Subtree Medium no
334 Increasing Triplet Subsequence Medium no
335 Self Crossing Hard no
336 Palindrome Pairs Hard no
337 House Robber III Medium no
338 Counting Bits Medium no
339 Nested List Weight Sum Easy no
340 Longest Substring with At Most K Distinct Characters Hard no
341 Flatten Nested List Iterator Medium no
342 Power of Four Easy no
343 Integer Break Medium no
344 Reverse String Easy no
345 Reverse Vowels of a String Easy no
346 Moving Average from Data Stream Easy no
347 Top K Frequent Elements Medium no
348 Design Tic-Tac-Toe Medium no
349 Intersection of Two Arrays Easy no
350 Intersection of Two Arrays II Easy no
351 Android Unlock Patterns Medium no
352 Data Stream as Disjoint Intervals Hard no
353 Design Snake Game Medium no
354 Russian Doll Envelopes Hard no
355 Design Twitter Medium no
356 Line Reflection Medium no
357 Count Numbers with Unique Digits Medium no
358 Rearrange String k Distance Apart Hard no
359 Logger Rate Limiter Easy no
360 Sort Transformed Array Medium no
361 Bomb Enemy Medium no
362 Design Hit Counter Medium no
363 Max Sum of Rectangle No Larger Than K Hard no
364 Nested List Weight Sum II Medium no
365 Water and Jug Problem Medium no
366 Find Leaves of Binary Tree Medium no
367 Valid Perfect Square Medium no
368 Largest Divisible Subset Medium no
369 Plus One Linked List Medium no
370 Range Addition Medium no
371 Sum of Two Integers Easy no
372 Super Pow Medium no
373 Find K Pairs with Smallest Sums Medium no
374 Guess Number Higher or Lower Easy no
375 Guess Number Higher or Lower II Medium no
376 Wiggle Subsequence Medium no
377 Combination Sum IV Medium no
378 Kth Smallest Element in a Sorted Matrix Medium no
379 Design Phone Directory Medium no
380 Insert Delete GetRandom O(1) Hard no
381 Insert Delete GetRandom O(1) - Duplicates allowed Hard no
382 Linked List Random Node Medium no
383 Ransom Note Easy no
384 Shuffle an Array Medium no
385 Mini Parser Medium no
386 Lexicographical Numbers Medium no
387 First Unique Character in a String Easy no
388 Longest Absolute File Path Medium no
389 Find the Difference Easy no
390 Elimination Game Medium no
391 Perfect Rectangle Hard no
392 Is Subsequence Medium no
393 UTF-8 Validation Medium no
394 Decode String Medium no
395 Longest Substring with At Least K Repeating Characters Medium no
396 Rotate Function Easy no
397 Integer Replacement Medium no
398 Random Pick Index Medium no
399 Evaluate Division Medium no
400 Nth Digit Easy no
401 Binary Watch Easy no
402 Remove K Digits Medium no
403 Frog Jump Hard no
404 Sum of Left Leaves Easy no
405 Convert a Number to Hexadecimal Easy no
406 Queue Reconstruction by Height Medium no
407 Trapping Rain Water II Hard no
408 Valid Word Abbreviation Easy no
409 Longest Palindrome Easy no
410 Split Array Largest Sum Hard no
411 Minimum Unique Word Abbreviation Hard no
412 Fizz Buzz Easy no
413 Arithmetic Slices Medium no
415 Add Strings Easy no
416 Partition Equal Subset Sum Medium no
417 Pacific Atlantic Water Flow Medium no
418 Sentence Screen Fitting Medium no
419 Battleships in a Board Medium no
423 Reconstruct Original Digits from English Medium no
434 Number of Segments in a String Easy no
437 Path Sum III Easy no
438 Find All Anagrams in a String Easy no
441 Arranging Coins Easy no
442 Find All Duplicates in an Array Medium no
443 String Compression Easy no
445 Add Two Numbers II Medium no
447 Number of Boomerangs Easy no
448 Find All Numbers Disappeared in an Array Easy no
451 Sort Characters By Frequency Medium no
453 Minimum Moves to Equal Array Elements Easy no
454 4Sum II Medium no
455 Assign Cookies Easy no
459 Repeated Substring Pattern Easy no
461 Hamming Distance Easy no
462 Minimum Moves to Equal Array Elements II Medium no
463 Island Perimeter Easy no
464 Can I Win Medium no
468 Validate IP Address Medium no
475 Heaters Easy no
476 Number Complement Easy no
477 Total Hamming Distance Medium no
479 Largest Palindrome Product Easy no
481 Magical String Medium no
482 License Key Formatting Medium no
485 Max Consecutive Ones Easy no
486 Predict the Winner Medium no
491 Increasing Subsequences Medium no
492 Construct the Rectangle Medium no
494 Target Sum Medium no
495 Teemo Attacking Medium no
496 Next Greater Element I Easy no
498 Diagonal Traverse Medium no
500 Keyboard Row Easy no
501 Find Mode in Binary Search Tree Medium no
503 Next Greater Element II Medium no
504 Base 7 Easy no
506 Relative Ranks Easy no
507 Perfect Number Easy no
508 Most Frequent Subtree Sum Medium no
513 Find Bottom Left Tree Value Medium no
515 Find Largest Value in Each Tree Row Medium no
516 Longest Palindromic Subsequence Medium no
521 Longest Uncommon Subsequence I Easy no
532 K-diff Pairs in an Array Easy no
537 Complex Number Multiplication Medium no
538 Convert BST to Greater Tree Easy no
541 Reverse String II Easy no
543 Diameter of Binary Tree Easy no
547 Friend Circles Medium no
551 Student Attendance Record I Easy no
553 Optimal Division Medium no
561 Array Partition I Easy no
563 Binary Tree Tilt Easy no
566 Reshape the Matrix Easy no
572 Subtree of Another Tree Easy no
575 Distribute Candies Easy no
581 Shortest Unsorted Continuous Subarray Easy no
599 Minimum Index Sum of Two Lists Easy no
617 Merge Two Binary Trees Easy no
628 Maximum Product of Three Numbers Easy no
633 Sum of Square Numbers Easy no
637 Average of Levels in Binary Tree Easy no
643 Maximum Average Subarray I Easy no
650 2 Keys Keyboard Medium no
653 Two Sum IV - Input is a BST Easy no
654 Maximum Binary Tree Medium no
655 Print Binary Tree Medium no
657 Judge Route Circle Easy no
671 Second Minimum Node In a Binary Tree Easy no
674 Longest Continuous Increasing Subsequence Easy no
695 Max Area of Island Easy no
709 To Lower Case Easy no
717 1-bit and 2-bit Characters Easy no
724 Find Pivot Index Easy no
728 Self Dividing Numbers Easy no
733 Flood Fill Easy no
739 Daily Temperatures Medium no
740 Delete and Earn Medium no
744 Find Smallest Letter Greater Than Target Easy no
746 Min Cost Climbing Stairs Easy no
747 Largest Number At Least Twice of Others Easy no
760 Find Anagram Mappings Easy no
762 Prime Number of Set Bits in Binary Representation Easy no
763 Partition Labels Medium no
766 Toeplitz Matrix Easy no
768 Max Chunks To Make Sorted II Hard no
769 Max Chunks To Make Sorted Medium no
771 Jewels and Stones Easy no
775 Global and Local Inversions Medium no
779 K-th Symbol in Grammar Medium no
804 Unique Morse Code Words Easy no
832 Flipping an Image Easy no