100 LeetCode Python Solutions: Interview Study Guide

Study 100 essential LeetCode interview problems with original Python solutions, pattern explanations, correctness arguments, complexity analysis, and edge cases. The main collection follows LeetCode’s official Top Interview study plan and adds the site’s earlier worked solutions.

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How to use this library

Choose a topic or difficulty in the searchable table below.
Open the official LeetCode prompt from the solution page and attempt it before reading the code.
Identify the reusable pattern: two pointers, sliding window, monotonic reasoning, graph traversal, topological sort, or another invariant.
Explain the complexity and correctness argument aloud before submitting.
Return later and implement the solution without looking.

Patterns worth mastering

Pattern	Recognize it when…
Two pointers	Input is sorted, endpoints constrain an optimum, or values must be compacted in place.
Sliding window	A contiguous range must satisfy a count, sum, or uniqueness condition.
Hash map / set	Fast membership, complements, frequencies, or previously seen states matter.
Stack	Nested structure, nearest unmatched items, or expression evaluation is involved.
Tree DFS / BFS	The answer decomposes by subtree or by level.
Graph traversal	Objects and relationships form connected components, paths, or dependency cycles.
Trie	Many word or prefix searches share the same character paths.

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103 solutions shown

#	Problem and solution	Difficulty	Topic	Pattern
1	LeetCode 1: Two Sum — Python Solution	Easy	Hashmap	complement lookup
2	LeetCode 2: Add Two Numbers — Python Solution	Medium	Linked List	digitwise carry
3	LeetCode 3: Longest Substring Without Repeating Characters — Python Solution	Medium	Sliding Window	last-seen sliding window
6	LeetCode 6: Zigzag Conversion — Python Solution	Medium	Array / String	row simulation
11	LeetCode 11: Container With Most Water — Python Solution	Medium	Two Pointers	greedy boundary movement
12	LeetCode 12: Integer to Roman — Python Solution	Medium	Array / String	greedy token emission
13	LeetCode 13: Roman to Integer — Python Solution	Easy	Array / String	look-ahead parsing
14	LeetCode 14: Longest Common Prefix — Python Solution	Easy	Array / String	vertical character scan
15	LeetCode 15: 3Sum — Python Solution	Medium	Two Pointers	sort plus two pointers
19	LeetCode 19: Remove Nth Node From End of List — Python Solution	Medium	Linked List	fixed-gap pointers
20	LeetCode 20: Valid Parentheses — Python Solution	Easy	Stack	expected-closer stack
21	LeetCode 21: Merge Two Sorted Lists — Python Solution	Easy	Linked List	dummy-tail merge
25	LeetCode 25: Reverse Nodes in k-Group — Python Solution	Hard	Linked List	group boundary reversal
26	LeetCode 26: Remove Duplicates from Sorted Array — Python Solution	Easy	Array / String	sorted write pointer
27	LeetCode 27: Remove Element — Python Solution	Easy	Array / String	write pointer
28	LeetCode 28: Find the Index of the First Occurrence in a String — Python Solution	Easy	Array / String	substring scan
30	LeetCode 30: Substring with Concatenation of All Words — Python Solution	Hard	Sliding Window	word-aligned windows
36	LeetCode 36: Valid Sudoku — Python Solution	Medium	Matrix	row-column-box sets
42	LeetCode 42: Trapping Rain Water — Python Solution	Hard	Array / String	two pointers with bounded sides
45	LeetCode 45: Jump Game II — Python Solution	Medium	Array / String	greedy BFS frontier
48	LeetCode 48: Rotate Image — Python Solution	Medium	Matrix	transpose then reverse rows
49	LeetCode 49: Group Anagrams — Python Solution	Medium	Hashmap	canonical frequency key
54	LeetCode 54: Spiral Matrix — Python Solution	Medium	Matrix	shrinking boundaries
55	LeetCode 55: Jump Game — Python Solution	Medium	Array / String	greedy reachability
56	LeetCode 56: Merge Intervals — Python Solution	Medium	Intervals	sorted sweep
57	LeetCode 57: Insert Interval — Python Solution	Medium	Intervals	three-phase interval scan
58	LeetCode 58: Length of Last Word — Python Solution	Easy	Array / String	reverse scan
61	LeetCode 61: Rotate List — Python Solution	Medium	Linked List	cycle then cut
68	LeetCode 68: Text Justification — Python Solution	Hard	Array / String	greedy line packing
71	LeetCode 71: Simplify Path — Python Solution	Medium	Stack	canonical path stack
73	LeetCode 73: Set Matrix Zeroes — Python Solution	Medium	Matrix	first-row marker compression
76	LeetCode 76: Minimum Window Substring — Python Solution	Hard	Sliding Window	deficit-count window
79	Leetcode 79 – Boggle Game / Word Search	Medium	Backtracking	depth-first search with backtracking
80	LeetCode 80: Remove Duplicates from Sorted Array II — Python Solution	Medium	Array / String	bounded write pointer
82	LeetCode 82: Remove Duplicates from Sorted List II — Python Solution	Medium	Linked List	run skipping
86	LeetCode 86: Partition List — Python Solution	Medium	Linked List	stable dual lists
88	LeetCode 88: Merge Sorted Array — Python Solution	Easy	Array / String	reverse two pointers
92	LeetCode 92: Reverse Linked List II — Python Solution	Medium	Linked List	head insertion reversal
98	LeetCode 98: Validate Binary Search Tree — Python Solution	Medium	Binary Search Tree	recursive value bounds
100	LeetCode 100: Same Tree — Python Solution	Easy	Binary Tree General	paired recursion
101	LeetCode 101: Symmetric Tree — Python Solution	Easy	Binary Tree General	mirror recursion
102	LeetCode 102: Binary Tree Level Order Traversal — Python Solution	Medium	Binary Tree BFS	breadth-first levels
103	LeetCode 103: Binary Tree Zigzag Level Order Traversal — Python Solution	Medium	Binary Tree BFS	alternating level output
104	LeetCode 104: Maximum Depth of Binary Tree — Python Solution	Easy	Binary Tree General	recursive postorder
105	LeetCode 105: Construct Binary Tree from Preorder and Inorder Traversal — Python Solution	Medium	Binary Tree General	indexed recursive partition
106	LeetCode 106: Construct Binary Tree from Inorder and Postorder Traversal — Python Solution	Medium	Binary Tree General	reverse postorder partition
112	LeetCode 112: Path Sum — Python Solution	Easy	Binary Tree General	remaining-sum DFS
114	LeetCode 114: Flatten Binary Tree to Linked List — Python Solution	Medium	Binary Tree General	reverse preorder threading
117	LeetCode 117: Populating Next Right Pointers in Each Node II — Python Solution	Medium	Binary Tree General	level linked-list construction
121	LeetCode 121: Best Time to Buy and Sell Stock — Python Solution	Easy	Array / String	running minimum
122	LeetCode 122: Best Time to Buy and Sell Stock II — Python Solution	Medium	Array / String	greedy positive differences
124	LeetCode 124: Binary Tree Maximum Path Sum — Python Solution	Hard	Binary Tree General	postorder gain DP
125	LeetCode 125: Valid Palindrome — Python Solution	Easy	Two Pointers	filtered two pointers
127	LeetCode 127: Word Ladder — Python Solution	Hard	Graph BFS	wildcard-neighbor BFS
128	LeetCode 128: Longest Consecutive Sequence — Python Solution	Medium	Hashmap	set sequence starts
129	LeetCode 129: Sum Root to Leaf Numbers — Python Solution	Medium	Binary Tree General	prefix-number DFS
130	LeetCode 130: Surrounded Regions — Python Solution	Medium	Graph General	boundary flood fill
133	LeetCode 133: Clone Graph — Python Solution	Medium	Graph General	DFS memoized cloning
134	LeetCode 134: Gas Station — Python Solution	Medium	Array / String	greedy restart
135	LeetCode 135: Candy — Python Solution	Hard	Array / String	two directional passes
138	LeetCode 138: Copy List with Random Pointer — Python Solution	Medium	Linked List	node-to-copy map
141	LeetCode 141: Linked List Cycle — Python Solution	Easy	Linked List	Floyd cycle detection
146	LeetCode 146: LRU Cache — Python Solution	Medium	Linked List	hash map plus doubly linked list
150	LeetCode 150: Evaluate Reverse Polish Notation — Python Solution	Medium	Stack	operand stack
151	LeetCode 151: Reverse Words in a String — Python Solution	Medium	Array / String	token normalization
155	LeetCode 155: Min Stack — Python Solution	Medium	Stack	paired minimum stack
167	LeetCode 167: Two Sum II – Input Array Is Sorted — Python Solution	Medium	Two Pointers	opposing two pointers
169	LeetCode 169: Majority Element — Python Solution	Easy	Array / String	Boyer-Moore voting
173	LeetCode 173: Binary Search Tree Iterator — Python Solution	Medium	Binary Tree General	lazy inorder stack
189	LeetCode 189: Rotate Array — Python Solution	Medium	Array / String	three reversals
199	LeetCode 199: Binary Tree Right Side View — Python Solution	Medium	Binary Tree BFS	level-order last node
200	LeetCode 200: Number of Islands — Python Solution	Medium	Graph General	grid flood fill
202	LeetCode 202: Happy Number — Python Solution	Easy	Hashmap	cycle detection
205	LeetCode 205: Isomorphic Strings — Python Solution	Easy	Hashmap	bidirectional mapping
207	LeetCode 207: Course Schedule — Python Solution	Medium	Graph General	Kahn topological sort
208	LeetCode 208: Implement Trie (Prefix Tree) — Python Solution	Medium	Trie	nested child maps
209	LeetCode 209: Minimum Size Subarray Sum — Python Solution	Medium	Sliding Window	positive sliding window
210	LeetCode 210: Course Schedule II — Python Solution	Medium	Graph General	topological ordering
211	LeetCode 211: Design Add and Search Words Data Structure — Python Solution	Medium	Trie	trie plus wildcard DFS
212	LeetCode 212: Word Search II — Python Solution	Hard	Trie	trie-guided board DFS
219	LeetCode 219: Contains Duplicate II — Python Solution	Easy	Hashmap	last-index tracking
222	LeetCode 222: Count Complete Tree Nodes — Python Solution	Medium	Binary Tree General	perfect-subtree detection
224	LeetCode 224: Basic Calculator — Python Solution	Hard	Stack	sign-context stack
226	LeetCode 226: Invert Binary Tree — Python Solution	Easy	Binary Tree General	recursive swap
228	LeetCode 228: Summary Ranges — Python Solution	Easy	Intervals	run detection
230	LeetCode 230: Kth Smallest Element in a BST — Python Solution	Medium	Binary Search Tree	iterative inorder
236	LeetCode 236: Lowest Common Ancestor of a Binary Tree — Python Solution	Medium	Binary Tree General	postorder target propagation
238	LeetCode 238: Product of Array Except Self — Python Solution	Medium	Array / String	prefix and suffix products
242	LeetCode 242: Valid Anagram — Python Solution	Easy	Hashmap	frequency equality
274	LeetCode 274: H-Index — Python Solution	Medium	Array / String	sorting and threshold scan
289	LeetCode 289: Game of Life — Python Solution	Medium	Matrix	encoded in-place transition
290	LeetCode 290: Word Pattern — Python Solution	Easy	Hashmap	bidirectional token mapping
380	LeetCode 380: Insert Delete GetRandom O(1) — Python Solution	Medium	Array / String	array plus index map
383	LeetCode 383: Ransom Note — Python Solution	Easy	Hashmap	frequency consumption
392	LeetCode 392: Is Subsequence — Python Solution	Easy	Two Pointers	subsequence pointer
399	LeetCode 399: Evaluate Division — Python Solution	Medium	Graph General	weighted graph search
433	LeetCode 433: Minimum Genetic Mutation — Python Solution	Medium	Graph BFS	single-character BFS
452	LeetCode 452: Minimum Number of Arrows to Burst Balloons — Python Solution	Medium	Intervals	greedy earliest end
530	LeetCode 530: Minimum Absolute Difference in BST — Python Solution	Easy	Binary Search Tree	inorder adjacent difference
637	LeetCode 637: Average of Levels in Binary Tree — Python Solution	Easy	Binary Tree BFS	level aggregation
733	Leetcode 733 – FloodFill	Easy	Graph General	grid flood fill
909	LeetCode 909: Snakes and Ladders — Python Solution	Medium	Graph BFS	board-index BFS
1768	Leetcode 1768 – Merge Strings Alternatively	Easy	Array / String	two-pointer interleaving

About these answers

Every implementation in the downloadable library is independently written and exercised by the included behavioral test suite. These pages deliberately avoid reproducing LeetCode’s full problem statements or examples; each solution links to the official prompt for the authoritative requirements and submission environment.