Stack LeetCode Problems: All 157,
With Python Solutions

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

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Easy: 22
Medium: 89
Hard: 46

1 pattern

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.

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python

Please analyze this context and help me answer the question or solve the problem.

Technical interview · 45 minsolution.pyShared editor · Python 3Longest substring without repeatsMediumGiven a string s, return the length of the longest contiguous substring in which every character is unique.s = "abcabcbb"
→ 31def length_of_longest_substring(s: str) -> int:2 last_seen = {}3 left = best = 04 for right, ch in enumerate(s):5 if ch in last_seen and last_seen[ch] >= left:

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

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GPT-6.1 Terra

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python

Please analyze this context and help me answer the question or solve the problem.

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

90%Subscription

python

Please analyze this context and help me answer the question or solve the problem.

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

Primary display

QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

Stack problems by difficulty

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

Easy (22)

Easy problems
No.	Problem	Topics
0020	Valid Parentheses	Stack, String
0094	Binary Tree Inorder Traversal	Stack, Tree, Depth-First Search +1
0144	Binary Tree Preorder Traversal	Stack, Tree, Depth-First Search +1
0145	Binary Tree Postorder Traversal	Stack, Tree, Depth-First Search +1
0225	Implement Stack using Queues	Stack, Design, Queue
0232	Implement Queue using Stacks	Stack, Design, Queue
0234	Palindrome Linked List	Stack, Recursion, Linked List +1
0496	Next Greater Element I	Stack, Array, Hash Table +1
0589	N-ary Tree Preorder Traversal	Stack, Tree, Depth-First Search
0590	N-ary Tree Postorder Traversal	Stack, Tree, Depth-First Search
0682	Baseball Game	Stack, Array, Simulation
0844	Backspace String Compare	Stack, Two Pointers, String +1
0897	Increasing Order Search Tree	Stack, Tree, Depth-First Search +2
1021	Remove Outermost Parentheses	Stack, String
1047	Remove All Adjacent Duplicates In String	Stack, String
1475	Final Prices With a Special Discount in a Shop	Stack, Array, Monotonic Stack
1544	Make The String Great	Stack, String
1598	Crawler Log Folder	Stack, Array, String
1614	Maximum Nesting Depth of the Parentheses	Stack, String
1700	Number of Students Unable to Eat Lunch	Stack, Queue, Array +1
2000	Reverse Prefix of Word	Stack, Two Pointers, String
2696	Minimum String Length After Removing Substrings	Stack, String, Simulation

Medium (89)

Medium problems
No.	Problem	Topics
0071	Simplify Path	Stack, String
0114	Flatten Binary Tree to Linked List	Stack, Tree, Depth-First Search +2
0143	Reorder List	Stack, Recursion, Linked List +1
0150	Evaluate Reverse Polish Notation	Stack, Array, Math
0155	Min Stack	Stack, Design
0173	Binary Search Tree Iterator	Stack, Tree, Design +3
0227	Basic Calculator II	Stack, Math, String
0316	Remove Duplicate Letters	Stack, Greedy, String +1
0331	Verify Preorder Serialization of a Binary Tree	Stack, Tree, String +1
0341	Flatten Nested List Iterator	Stack, Tree, Depth-First Search +3
0385	Mini Parser	Stack, Depth-First Search, String
0388	Longest Absolute File Path	Stack, Depth-First Search, String
0394	Decode String	Stack, Recursion, String
0402	Remove K Digits	Stack, Greedy, String +1
0445	Add Two Numbers II	Stack, Linked List, Math
0456	132 Pattern	Stack, Array, Binary Search +2
0503	Next Greater Element II	Stack, Array, Monotonic Stack
0581	Shortest Unsorted Continuous Subarray	Stack, Greedy, Array +3
0636	Exclusive Time of Functions	Stack, Array
0654	Maximum Binary Tree	Stack, Tree, Array +3
0678	Valid Parenthesis String	Stack, Greedy, String +1
0735	Asteroid Collision	Stack, Array, Simulation
0739	Daily Temperatures	Stack, Array, Monotonic Stack
0769	Max Chunks To Make Sorted	Stack, Greedy, Array +2
0853	Car Fleet	Stack, Array, Sorting +1
0856	Score of Parentheses	Stack, String
0880	Decoded String at Index	Stack, String
0901	Online Stock Span	Stack, Design, Data Stream +1
0907	Sum of Subarray Minimums	Stack, Array, Dynamic Programming +1
0921	Minimum Add to Make Parentheses Valid	Stack, Greedy, String
0946	Validate Stack Sequences	Stack, Array, Simulation
0962	Maximum Width Ramp	Stack, Array, Two Pointers +1
1003	Check If Word Is Valid After Substitutions	Stack, String
1006	Clumsy Factorial	Stack, Math, Simulation
1008	Construct Binary Search Tree from Preorder Traversal	Stack, Tree, Binary Search Tree +3
1019	Next Greater Node In Linked List	Stack, Array, Linked List +1
1081	Smallest Subsequence of Distinct Characters	Stack, Greedy, String +1
1111	Maximum Nesting Depth of Two Valid Parentheses Strings	Stack, String
1124	Longest Well-Performing Interval	Stack, Array, Hash Table +2
1130	Minimum Cost Tree From Leaf Values	Stack, Greedy, Array +2
1190	Reverse Substrings Between Each Pair of Parentheses	Stack, String
1209	Remove All Adjacent Duplicates in String II	Stack, String
1249	Minimum Remove to Make Valid Parentheses	Stack, String
1381	Design a Stack With Increment Operation	Stack, Design, Array
1441	Build an Array With Stack Operations	Stack, Array, Simulation
1472	Design Browser History	Stack, Design, Array +3
1504	Count Submatrices With All Ones	Stack, Array, Dynamic Programming +2
1541	Minimum Insertions to Balance a Parentheses String	Stack, Greedy, String
1574	Shortest Subarray to be Removed to Make Array Sorted	Stack, Array, Two Pointers +2
1653	Minimum Deletions to Make String Balanced	Stack, String, Dynamic Programming
1673	Find the Most Competitive Subsequence	Stack, Greedy, Array +1
1717	Maximum Score From Removing Substrings	Stack, Greedy, String
1856	Maximum Subarray Min-Product	Stack, Array, Prefix Sum +1
1910	Remove All Occurrences of a Substring	Stack, String, Simulation
1963	Minimum Number of Swaps to Make the String Balanced	Stack, Greedy, Two Pointers +1
1996	The Number of Weak Characters in the Game	Stack, Greedy, Array +2
2104	Sum of Subarray Ranges	Stack, Array, Monotonic Stack
2116	Check if a Parentheses String Can Be Valid	Stack, Greedy, String
2130	Maximum Twin Sum of a Linked List	Stack, Linked List, Two Pointers
2211	Count Collisions on a Road	Stack, String, Simulation
2216	Minimum Deletions to Make Array Beautiful	Stack, Greedy, Array
2289	Steps to Make Array Non-decreasing	Stack, Array, Linked List +1
2375	Construct Smallest Number From DI String	Stack, Greedy, String +1
2390	Removing Stars From a String	Stack, String, Simulation
2434	Using a Robot to Print the Lexicographically Smallest String	Stack, Greedy, Hash Table +1
2487	Remove Nodes From Linked List	Stack, Recursion, Linked List +1
2645	Minimum Additions to Make Valid String	Stack, Greedy, String +1
2816	Double a Number Represented as a Linked List	Stack, Linked List, Math
2865	Beautiful Towers I	Stack, Array, Monotonic Stack
2866	Beautiful Towers II	Stack, Array, Monotonic Stack
0255	Verify Preorder Sequence in Binary Search TreePremium	Stack, Tree, Binary Search Tree +4
0364	Nested List Weight Sum IIPremium	Stack, Depth-First Search, Breadth-First Search
0426	Convert Binary Search Tree to Sorted Doubly Linked ListPremium	Stack, Tree, Depth-First Search +4
0439	Ternary Expression ParserPremium	Stack, Recursion, String
0484	Find PermutationPremium	Stack, Greedy, Array +1
0536	Construct Binary Tree from StringPremium	Stack, Tree, Depth-First Search +2
1087	Brace ExpansionPremium	Stack, Breadth-First Search, String +2
1214	Two Sum BSTsPremium	Stack, Tree, Depth-First Search +4
1265	Print Immutable Linked List in ReversePremium	Stack, Recursion, Linked List +1
1586	Binary Search Tree Iterator IIPremium	Stack, Tree, Design +3
1628	Design an Expression Tree With Evaluate FunctionPremium	Stack, Tree, Design +3
1762	Buildings With an Ocean ViewPremium	Stack, Array, Monotonic Stack
1950	Maximum of Minimum Values in All SubarraysPremium	Stack, Array, Monotonic Stack
2282	Number of People That Can Be Seen in a GridPremium	Stack, Array, Matrix +1
2297	Jump Game VIIIPremium	Stack, Graph, Array +3
2345	Finding the Number of Visible MountainsPremium	Stack, Array, Sorting +1
2764	Is Array a Preorder of Some ‌Binary TreePremium	Stack, Tree, Depth-First Search +1
2832	Maximal Range That Each Element Is Maximum in ItPremium	Stack, Array, Monotonic Stack
2863	Maximum Length of Semi-Decreasing SubarraysPremium	Stack, Array, Sorting +1

Hard (46)

Hard problems
No.	Problem	Topics
0032	Longest Valid Parentheses	Stack, String, Dynamic Programming
0042	Trapping Rain Water	Stack, Array, Two Pointers +2
0084	Largest Rectangle in Histogram	Stack, Array, Monotonic Stack
0085	Maximal Rectangle	Stack, Array, Dynamic Programming +2
0224	Basic Calculator	Stack, Recursion, Math +1
0321	Create Maximum Number	Stack, Greedy, Array +2
0488	Zuma Game	Stack, Breadth-First Search, Memoization +2
0591	Tag Validator	Stack, String
0736	Parse Lisp Expression	Stack, Recursion, Hash Table +1
0768	Max Chunks To Make Sorted II	Stack, Greedy, Array +2
0895	Maximum Frequency Stack	Stack, Design, Hash Table +1
0936	Stamping The Sequence	Stack, Greedy, Queue +1
0975	Odd Even Jump	Stack, Array, Dynamic Programming +3
1096	Brace Expansion II	Stack, Breadth-First Search, Hash Table +3
1106	Parsing A Boolean Expression	Stack, Recursion, String
1172	Dinner Plate Stacks	Stack, Design, Hash Table +1
1526	Minimum Number of Increments on Subarrays to Form a Target Array	Stack, Greedy, Array +2
1776	Car Fleet II	Stack, Array, Math +2
1793	Maximum Score of a Good Subarray	Stack, Array, Two Pointers +2
1944	Number of Visible People in a Queue	Stack, Array, Monotonic Stack
2019	The Score of Students Solving Math Expression	Stack, Memoization, Array +4
2197	Replace Non-Coprime Numbers in Array	Stack, Array, Math +1
2281	Sum of Total Strength of Wizards	Stack, Array, Prefix Sum +1
2296	Design a Text Editor	Stack, Design, Linked List +3
2334	Subarray With Elements Greater Than Varying Threshold	Stack, Union Find, Array +1
2454	Next Greater Element IV	Stack, Array, Binary Search +3
2589	Minimum Time to Complete All Tasks	Stack, Greedy, Array +2
2617	Minimum Number of Visited Cells in a Grid	Stack, Breadth-First Search, Union Find +5
2736	Maximum Sum Queries	Stack, Binary Indexed Tree, Segment Tree +4
2751	Robot Collisions	Stack, Array, Sorting +1
2813	Maximum Elegance of a K-Length Subsequence	Stack, Greedy, Array +3
2818	Apply Operations to Maximize Score	Stack, Greedy, Array +4
2940	Find Building Where Alice and Bob Can Meet	Stack, Binary Indexed Tree, Segment Tree +4
2945	Find Maximum Non-decreasing Array Length	Stack, Queue, Array +4
0272	Closest Binary Search Tree Value IIPremium	Stack, Tree, Depth-First Search +4
0716	Max StackPremium	Stack, Design, Linked List +2
0726	Number of Atoms	Stack, Hash Table, String +1
0770	Basic Calculator IV	Stack, Recursion, Hash Table +2
0772	Basic Calculator IIIPremium	Stack, Recursion, Math +1
1063	Number of Valid SubarraysPremium	Stack, Array, Monotonic Stack
1597	Build Binary Expression Tree From Infix ExpressionPremium	Stack, Tree, String +1
1896	Minimum Cost to Change the Final Value of Expression	Stack, Math, String +1
2030	Smallest K-Length Subsequence With Occurrences of a Letter	Stack, Greedy, String +1
2254	Design Video Sharing PlatformPremium	Stack, Design, Hash Table +1
2355	Maximum Number of Books You Can TakePremium	Stack, Array, Dynamic Programming +1
2524	Maximum Frequency Score of a SubarrayPremium	Stack, Array, Hash Table +2

How Stack problems are solved

When the Stack problem arrives live.

Approach

Approach

Approach

Stack problems by difficulty

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Medium (89)89

Hard (46)46

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