20. Valid Parentheses
155. Min Stack
225. Implement Stack using Queues
232. Implement Queue using Stacks
496. Next Greater Element I
581. Shortest Unsorted Continuous Subarray
1047. Remove All Adjacent Duplicates In String
56. Merge Intervals
71. Simplify Path
150. Evaluate Reverse Polish Notation
173. Binary Search Tree Iterator
227. Basic Calculator II
394. Decode String
402. Remove K Digits
1019. Next Greater Node In Linked List
1081. Smallest Subsequence of Distinct Characters
32. Longest Valid Parentheses
57. Insert Interval
84. Largest Rectangle in Histogram
85. Maximal Rectangle
316. Remove Duplicate Letters