Stack LeetCode Problems: All 157, With Python Solutions

Every problem in this library that LeetCode tags Stack — 157 in total, 126 of them with a complete Python solution, a worked example and the time and space complexity of the approach.

157 problems
22 Easy
89 Medium
46 Hard

How Stack problems are solved

A tag names the subject, not the method. These pattern hubs cover the techniques that actually solve Stack problems — each one explains the approach, gives a Python template and states its complexity.

Stack — When the most recent unresolved thing is the one that matters, use a stack.

Stack problems by difficulty

Problems with a complete Python solution are listed first, then by ascending problem number.

Easy (22)

#	Problem	Difficulty	Topics
20	Valid Parentheses	Easy	Stack, String
94	Binary Tree Inorder Traversal	Easy	Stack, Tree, Depth-First Search +1
144	Binary Tree Preorder Traversal	Easy	Stack, Tree, Depth-First Search +1
145	Binary Tree Postorder Traversal	Easy	Stack, Tree, Depth-First Search +1
225	Implement Stack using Queues	Easy	Stack, Design, Queue
232	Implement Queue using Stacks	Easy	Stack, Design, Queue
234	Palindrome Linked List	Easy	Stack, Recursion, Linked List +1
496	Next Greater Element I	Easy	Stack, Array, Hash Table +1
589	N-ary Tree Preorder Traversal	Easy	Stack, Tree, Depth-First Search
590	N-ary Tree Postorder Traversal	Easy	Stack, Tree, Depth-First Search
682	Baseball Game	Easy	Stack, Array, Simulation
844	Backspace String Compare	Easy	Stack, Two Pointers, String +1
897	Increasing Order Search Tree	Easy	Stack, Tree, Depth-First Search +2
1021	Remove Outermost Parentheses	Easy	Stack, String
1047	Remove All Adjacent Duplicates In String	Easy	Stack, String
1475	Final Prices With a Special Discount in a Shop	Easy	Stack, Array, Monotonic Stack
1544	Make The String Great	Easy	Stack, String
1598	Crawler Log Folder	Easy	Stack, Array, String
1614	Maximum Nesting Depth of the Parentheses	Easy	Stack, String
1700	Number of Students Unable to Eat Lunch	Easy	Stack, Queue, Array +1
2000	Reverse Prefix of Word	Easy	Stack, Two Pointers, String
2696	Minimum String Length After Removing Substrings	Easy	Stack, String, Simulation

Medium (89)

#	Problem	Difficulty	Topics
71	Simplify Path	Medium	Stack, String
114	Flatten Binary Tree to Linked List	Medium	Stack, Tree, Depth-First Search +2
143	Reorder List	Medium	Stack, Recursion, Linked List +1
150	Evaluate Reverse Polish Notation	Medium	Stack, Array, Math
155	Min Stack	Medium	Stack, Design
173	Binary Search Tree Iterator	Medium	Stack, Tree, Design +3
227	Basic Calculator II	Medium	Stack, Math, String
316	Remove Duplicate Letters	Medium	Stack, Greedy, String +1
331	Verify Preorder Serialization of a Binary Tree	Medium	Stack, Tree, String +1
341	Flatten Nested List Iterator	Medium	Stack, Tree, Depth-First Search +3
385	Mini Parser	Medium	Stack, Depth-First Search, String
388	Longest Absolute File Path	Medium	Stack, Depth-First Search, String
394	Decode String	Medium	Stack, Recursion, String
402	Remove K Digits	Medium	Stack, Greedy, String +1
445	Add Two Numbers II	Medium	Stack, Linked List, Math
456	132 Pattern	Medium	Stack, Array, Binary Search +2
503	Next Greater Element II	Medium	Stack, Array, Monotonic Stack
581	Shortest Unsorted Continuous Subarray	Medium	Stack, Greedy, Array +3
636	Exclusive Time of Functions	Medium	Stack, Array
654	Maximum Binary Tree	Medium	Stack, Tree, Array +3
678	Valid Parenthesis String	Medium	Stack, Greedy, String +1
735	Asteroid Collision	Medium	Stack, Array, Simulation
739	Daily Temperatures	Medium	Stack, Array, Monotonic Stack
769	Max Chunks To Make Sorted	Medium	Stack, Greedy, Array +2
853	Car Fleet	Medium	Stack, Array, Sorting +1
856	Score of Parentheses	Medium	Stack, String
880	Decoded String at Index	Medium	Stack, String
901	Online Stock Span	Medium	Stack, Design, Data Stream +1
907	Sum of Subarray Minimums	Medium	Stack, Array, Dynamic Programming +1
921	Minimum Add to Make Parentheses Valid	Medium	Stack, Greedy, String
946	Validate Stack Sequences	Medium	Stack, Array, Simulation
962	Maximum Width Ramp	Medium	Stack, Array, Two Pointers +1
1003	Check If Word Is Valid After Substitutions	Medium	Stack, String
1006	Clumsy Factorial	Medium	Stack, Math, Simulation
1008	Construct Binary Search Tree from Preorder Traversal	Medium	Stack, Tree, Binary Search Tree +3
1019	Next Greater Node In Linked List	Medium	Stack, Array, Linked List +1
1081	Smallest Subsequence of Distinct Characters	Medium	Stack, Greedy, String +1
1111	Maximum Nesting Depth of Two Valid Parentheses Strings	Medium	Stack, String
1124	Longest Well-Performing Interval	Medium	Stack, Array, Hash Table +2
1130	Minimum Cost Tree From Leaf Values	Medium	Stack, Greedy, Array +2
1190	Reverse Substrings Between Each Pair of Parentheses	Medium	Stack, String
1209	Remove All Adjacent Duplicates in String II	Medium	Stack, String
1249	Minimum Remove to Make Valid Parentheses	Medium	Stack, String
1381	Design a Stack With Increment Operation	Medium	Stack, Design, Array
1441	Build an Array With Stack Operations	Medium	Stack, Array, Simulation
1472	Design Browser History	Medium	Stack, Design, Array +3
1504	Count Submatrices With All Ones	Medium	Stack, Array, Dynamic Programming +2
1541	Minimum Insertions to Balance a Parentheses String	Medium	Stack, Greedy, String
1574	Shortest Subarray to be Removed to Make Array Sorted	Medium	Stack, Array, Two Pointers +2
1653	Minimum Deletions to Make String Balanced	Medium	Stack, String, Dynamic Programming
1673	Find the Most Competitive Subsequence	Medium	Stack, Greedy, Array +1
1717	Maximum Score From Removing Substrings	Medium	Stack, Greedy, String
1856	Maximum Subarray Min-Product	Medium	Stack, Array, Prefix Sum +1
1910	Remove All Occurrences of a Substring	Medium	Stack, String, Simulation
1963	Minimum Number of Swaps to Make the String Balanced	Medium	Stack, Greedy, Two Pointers +1
1996	The Number of Weak Characters in the Game	Medium	Stack, Greedy, Array +2
2104	Sum of Subarray Ranges	Medium	Stack, Array, Monotonic Stack
2116	Check if a Parentheses String Can Be Valid	Medium	Stack, Greedy, String
2130	Maximum Twin Sum of a Linked List	Medium	Stack, Linked List, Two Pointers
2211	Count Collisions on a Road	Medium	Stack, String, Simulation
2216	Minimum Deletions to Make Array Beautiful	Medium	Stack, Greedy, Array
2289	Steps to Make Array Non-decreasing	Medium	Stack, Array, Linked List +1
2375	Construct Smallest Number From DI String	Medium	Stack, Greedy, String +1
2390	Removing Stars From a String	Medium	Stack, String, Simulation
2434	Using a Robot to Print the Lexicographically Smallest String	Medium	Stack, Greedy, Hash Table +1
2487	Remove Nodes From Linked List	Medium	Stack, Recursion, Linked List +1
2645	Minimum Additions to Make Valid String	Medium	Stack, Greedy, String +1
2816	Double a Number Represented as a Linked List	Medium	Stack, Linked List, Math
2865	Beautiful Towers I	Medium	Stack, Array, Monotonic Stack
2866	Beautiful Towers II	Medium	Stack, Array, Monotonic Stack
255	Verify Preorder Sequence in Binary Search TreePremium	Medium	Stack, Tree, Binary Search Tree +4
364	Nested List Weight Sum IIPremium	Medium	Stack, Depth-First Search, Breadth-First Search
426	Convert Binary Search Tree to Sorted Doubly Linked ListPremium	Medium	Stack, Tree, Depth-First Search +4
439	Ternary Expression ParserPremium	Medium	Stack, Recursion, String
484	Find PermutationPremium	Medium	Stack, Greedy, Array +1
536	Construct Binary Tree from StringPremium	Medium	Stack, Tree, Depth-First Search +2
1087	Brace ExpansionPremium	Medium	Stack, Breadth-First Search, String +2
1214	Two Sum BSTsPremium	Medium	Stack, Tree, Depth-First Search +4
1265	Print Immutable Linked List in ReversePremium	Medium	Stack, Recursion, Linked List +1
1586	Binary Search Tree Iterator IIPremium	Medium	Stack, Tree, Design +3
1628	Design an Expression Tree With Evaluate FunctionPremium	Medium	Stack, Tree, Design +3
1762	Buildings With an Ocean ViewPremium	Medium	Stack, Array, Monotonic Stack
1950	Maximum of Minimum Values in All SubarraysPremium	Medium	Stack, Array, Monotonic Stack
2282	Number of People That Can Be Seen in a GridPremium	Medium	Stack, Array, Matrix +1
2297	Jump Game VIIIPremium	Medium	Stack, Graph, Array +3
2345	Finding the Number of Visible MountainsPremium	Medium	Stack, Array, Sorting +1
2764	Is Array a Preorder of Some ‌Binary TreePremium	Medium	Stack, Tree, Depth-First Search +1
2832	Maximal Range That Each Element Is Maximum in ItPremium	Medium	Stack, Array, Monotonic Stack
2863	Maximum Length of Semi-Decreasing SubarraysPremium	Medium	Stack, Array, Sorting +1

Hard (46)

#	Problem	Difficulty	Topics
32	Longest Valid Parentheses	Hard	Stack, String, Dynamic Programming
42	Trapping Rain Water	Hard	Stack, Array, Two Pointers +2
84	Largest Rectangle in Histogram	Hard	Stack, Array, Monotonic Stack
85	Maximal Rectangle	Hard	Stack, Array, Dynamic Programming +2
224	Basic Calculator	Hard	Stack, Recursion, Math +1
321	Create Maximum Number	Hard	Stack, Greedy, Array +2
488	Zuma Game	Hard	Stack, Breadth-First Search, Memoization +2
591	Tag Validator	Hard	Stack, String
736	Parse Lisp Expression	Hard	Stack, Recursion, Hash Table +1
768	Max Chunks To Make Sorted II	Hard	Stack, Greedy, Array +2
895	Maximum Frequency Stack	Hard	Stack, Design, Hash Table +1
936	Stamping The Sequence	Hard	Stack, Greedy, Queue +1
975	Odd Even Jump	Hard	Stack, Array, Dynamic Programming +3
1096	Brace Expansion II	Hard	Stack, Breadth-First Search, Hash Table +3
1106	Parsing A Boolean Expression	Hard	Stack, Recursion, String
1172	Dinner Plate Stacks	Hard	Stack, Design, Hash Table +1
1526	Minimum Number of Increments on Subarrays to Form a Target Array	Hard	Stack, Greedy, Array +2
1776	Car Fleet II	Hard	Stack, Array, Math +2
1793	Maximum Score of a Good Subarray	Hard	Stack, Array, Two Pointers +2
1944	Number of Visible People in a Queue	Hard	Stack, Array, Monotonic Stack
2019	The Score of Students Solving Math Expression	Hard	Stack, Memoization, Array +4
2197	Replace Non-Coprime Numbers in Array	Hard	Stack, Array, Math +1
2281	Sum of Total Strength of Wizards	Hard	Stack, Array, Prefix Sum +1
2296	Design a Text Editor	Hard	Stack, Design, Linked List +3
2334	Subarray With Elements Greater Than Varying Threshold	Hard	Stack, Union Find, Array +1
2454	Next Greater Element IV	Hard	Stack, Array, Binary Search +3
2589	Minimum Time to Complete All Tasks	Hard	Stack, Greedy, Array +2
2617	Minimum Number of Visited Cells in a Grid	Hard	Stack, Breadth-First Search, Union Find +5
2736	Maximum Sum Queries	Hard	Stack, Binary Indexed Tree, Segment Tree +4
2751	Robot Collisions	Hard	Stack, Array, Sorting +1
2813	Maximum Elegance of a K-Length Subsequence	Hard	Stack, Greedy, Array +3
2818	Apply Operations to Maximize Score	Hard	Stack, Greedy, Array +4
2940	Find Building Where Alice and Bob Can Meet	Hard	Stack, Binary Indexed Tree, Segment Tree +4
2945	Find Maximum Non-decreasing Array Length	Hard	Stack, Queue, Array +4
272	Closest Binary Search Tree Value IIPremium	Hard	Stack, Tree, Depth-First Search +4
716	Max StackPremium	Hard	Stack, Design, Linked List +2
726	Number of Atoms	Hard	Stack, Hash Table, String +1
770	Basic Calculator IV	Hard	Stack, Recursion, Hash Table +2
772	Basic Calculator IIIPremium	Hard	Stack, Recursion, Math +1
1063	Number of Valid SubarraysPremium	Hard	Stack, Array, Monotonic Stack
1597	Build Binary Expression Tree From Infix ExpressionPremium	Hard	Stack, Tree, String +1
1896	Minimum Cost to Change the Final Value of Expression	Hard	Stack, Math, String +1
2030	Smallest K-Length Subsequence With Occurrences of a Letter	Hard	Stack, Greedy, String +1
2254	Design Video Sharing PlatformPremium	Hard	Stack, Design, Hash Table +1
2355	Maximum Number of Books You Can TakePremium	Hard	Stack, Array, Dynamic Programming +1
2524	Maximum Frequency Score of a SubarrayPremium	Hard	Stack, Array, Hash Table +2

When the Stack problem arrives live

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