Stack

|20. Valid Parentheses | [Go]({{< relref "/ChapterFour/0020.Valid-Parentheses.md" >}})| Easy | O(log n)| O(1)||
|42. Trapping Rain Water | [Go]({{< relref "/ChapterFour/0042.Trapping-Rain-Water.md" >}})| Hard | O(n)| O(1)|❤️|
|71. Simplify Path | [Go]({{< relref "/ChapterFour/0071.Simplify-Path.md" >}})| Medium | O(n)| O(n)|❤️|
|84. Largest Rectangle in Histogram | [Go]({{< relref "/ChapterFour/0084.Largest-Rectangle-in-Histogram.md" >}})| Medium | O(n)| O(n)|❤️|
|94. Binary Tree Inorder Traversal | [Go]({{< relref "/ChapterFour/0094.Binary-Tree-Inorder-Traversal.md" >}})| Medium | O(n)| O(1)||
|103. Binary Tree Zigzag Level Order Traversal | [Go]({{< relref "/ChapterFour/0103.Binary-Tree-Zigzag-Level-Order-Traversal.md" >}})| Medium | O(n)| O(n)||
|144. Binary Tree Preorder Traversal | [Go]({{< relref "/ChapterFour/0144.Binary-Tree-Preorder-Traversal.md" >}})| Medium | O(n)| O(1)||
|145. Binary Tree Postorder Traversal | [Go]({{< relref "/ChapterFour/0145.Binary-Tree-Postorder-Traversal.md" >}})| Hard | O(n)| O(1)||
|150. Evaluate Reverse Polish Notation | [Go]({{< relref "/ChapterFour/0150.Evaluate-Reverse-Polish-Notation.md" >}})| Medium | O(n)| O(1)||
|155. Min Stack | [Go]({{< relref "/ChapterFour/0155.Min-Stack.md" >}})| Easy | O(n)| O(n)||
|173. Binary Search Tree Iterator | [Go]({{< relref "/ChapterFour/0173.Binary-Search-Tree-Iterator.md" >}})| Medium | O(n)| O(1)||
|224. Basic Calculator | [Go]({{< relref "/ChapterFour/0224.Basic-Calculator.md" >}})| Hard | O(n)| O(n)||
|225. Implement Stack using Queues | [Go]({{< relref "/ChapterFour/0225.Implement-Stack-using-Queues.md" >}})| Easy | O(n)| O(n)||
|232. Implement Queue using Stacks | [Go]({{< relref "/ChapterFour/0232.Implement-Queue-using-Stacks.md" >}})| Easy | O(n)| O(n)||
|331. Verify Preorder Serialization of a Binary Tree | [Go]({{< relref "/ChapterFour/0331.Verify-Preorder-Serialization-of-a-Binary-Tree.md" >}})| Medium | O(n)| O(1)||
|394. Decode String  | [Go]({{< relref "/ChapterFour/0394.Decode-String.md" >}})| Medium | O(n)| O(n)||
|402. Remove K Digits | [Go]({{< relref "/ChapterFour/0402.Remove-K-Digits.md" >}})| Medium | O(n)| O(1)||
|456. 132 Pattern  | [Go]({{< relref "/ChapterFour/0456.132-Pattern.md" >}})| Medium | O(n)| O(n)||
|496. Next Greater Element I | [Go]({{< relref "/ChapterFour/0496.Next-Greater-Element-I.md" >}})| Easy | O(n)| O(n)||
|503. Next Greater Element II  | [Go]({{< relref "/ChapterFour/0503.Next-Greater-Element-II.md" >}})| Medium | O(n)| O(n)||
|636. Exclusive Time of Functions | [Go]({{< relref "/ChapterFour/0636.Exclusive-Time-of-Functions.md" >}})| Medium | O(n)| O(n)||
|682. Baseball Game  | [Go]({{< relref "/ChapterFour/0682.Baseball-Game.md" >}})| Easy | O(n)| O(n)||
|726. Number of Atoms | [Go]({{< relref "/ChapterFour/0726.Number-of-Atoms.md" >}})| Hard | O(n)| O(n) |❤️|
|735. Asteroid Collision | [Go]({{< relref "/ChapterFour/0735.Asteroid-Collision.md" >}})| Medium | O(n)| O(n) ||
|739. Daily Temperatures | [Go]({{< relref "/ChapterFour/0739.Daily-Temperatures.md" >}})| Medium | O(n)| O(n) ||
|844. Backspace String Compare | [Go]({{< relref "/ChapterFour/0844.Backspace-String-Compare.md" >}})| Easy | O(n)| O(n) ||
|856. Score of Parentheses  | [Go]({{< relref "/ChapterFour/0856.Score-of-Parentheses.md" >}})| Medium | O(n)| O(n)||
|880. Decoded String at Index | [Go]({{< relref "/ChapterFour/0880.Decoded-String-at-Index.md" >}})| Medium | O(n)| O(n)||
|895. Maximum Frequency Stack | [Go]({{< relref "/ChapterFour/0895.Maximum-Frequency-Stack.md" >}})| Hard | O(n)| O(n)  ||
|901. Online Stock Span | [Go]({{< relref "/ChapterFour/0901.Online-Stock-Span.md" >}})| Medium | O(n)| O(n)  ||
|907. Sum of Subarray Minimums | [Go]({{< relref "/ChapterFour/0907.Sum-of-Subarray-Minimums.md" >}})| Medium | O(n)| O(n)|❤️|
|921. Minimum Add to Make Parentheses Valid | [Go]({{< relref "/ChapterFour/0921.Minimum-Add-to-Make-Parentheses-Valid.md" >}})| Medium | O(n)| O(n)||
|946. Validate Stack Sequences | [Go]({{< relref "/ChapterFour/0946.Validate-Stack-Sequences.md" >}})| Medium | O(n)| O(n)||
|1003. Check If Word Is Valid After Substitutions | [Go]({{< relref "/ChapterFour/1003.Check-If-Word-Is-Valid-After-Substitutions.md" >}})| Medium | O(n)| O(1)||
|1019. Next Greater Node In Linked List | [Go]({{< relref "/ChapterFour/1019.Next-Greater-Node-In-Linked-List.md" >}})| Medium | O(n)| O(1)||
|1021. Remove Outermost Parentheses | [Go]({{< relref "/ChapterFour/1021.Remove-Outermost-Parentheses.md" >}})| Medium | O(n)| O(1)||
|1047. Remove All Adjacent Duplicates In String | [Go]({{< relref "/ChapterFour/1047.Remove-All-Adjacent-Duplicates-In-String.md" >}})| Medium | O(n)| O(1)||