String LeetCode Problems: All 672,
With Python Solutions

Every problem in this library that LeetCode tags String. 568 of them carry a complete Python solution, a worked example and the time and space complexity of the approach.

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Easy: 196
Medium: 335
Hard: 141

String problems by difficulty

Showing the first 200 of 672 problems. Problems with a complete Python solution are listed first, then by ascending problem number.

Easy (55)

Easy problems
No.	Problem	Topics
0013	Roman to Integer	Hash Table, Math, String
0014	Longest Common Prefix	Trie, Array, String
0020	Valid Parentheses	Stack, String
0028	Find the Index of the First Occurrence in a String	Two Pointers, String, String Matching
0058	Length of Last Word	String
0067	Add Binary	Bit Manipulation, Math, String +1
0125	Valid Palindrome	Two Pointers, String
0168	Excel Sheet Column Title	Math, String
0171	Excel Sheet Column Number	Math, String
0205	Isomorphic Strings	Hash Table, String
0242	Valid Anagram	Hash Table, String, Sorting
0257	Binary Tree Paths	Tree, Depth-First Search, String +2
0290	Word Pattern	Hash Table, String
0344	Reverse String	Two Pointers, String
0345	Reverse Vowels of a String	Two Pointers, String
0383	Ransom Note	Hash Table, String, Counting
0387	First Unique Character in a String	Queue, Hash Table, String +1
0389	Find the Difference	Bit Manipulation, Hash Table, String +1
0392	Is Subsequence	Two Pointers, String, Dynamic Programming
0405	Convert a Number to Hexadecimal	Bit Manipulation, Math, String
0409	Longest Palindrome	Greedy, Hash Table, String
0412	Fizz Buzz	Math, String, Simulation
0415	Add Strings	Math, String, Simulation
0434	Number of Segments in a String	String
0459	Repeated Substring Pattern	String, String Matching
0482	License Key Formatting	String
0500	Keyboard Row	Array, Hash Table, String
0504	Base 7	Math, String
0520	Detect Capital	String
0521	Longest Uncommon Subsequence I	String
0541	Reverse String II	Two Pointers, String
0551	Student Attendance Record I	String
0557	Reverse Words in a String III	Two Pointers, String
0599	Minimum Index Sum of Two Lists	Array, Hash Table, String
0657	Robot Return to Origin	String, Simulation
0680	Valid Palindrome II	Greedy, Two Pointers, String
0696	Count Binary Substrings	Two Pointers, String
0709	To Lower Case	String
0748	Shortest Completing Word	Array, Hash Table, String
0771	Jewels and Stones	Hash Table, String
0796	Rotate String	String, String Matching
0804	Unique Morse Code Words	Array, Hash Table, String
0806	Number of Lines To Write String	Array, String
0819	Most Common Word	Array, Hash Table, String +1
0821	Shortest Distance to a Character	Array, Two Pointers, String
0824	Goat Latin	String
0830	Positions of Large Groups	String
0844	Backspace String Compare	Stack, Two Pointers, String +1
0859	Buddy Strings	Hash Table, String
0884	Uncommon Words from Two Sentences	Hash Table, String, Counting
0917	Reverse Only Letters	Two Pointers, String
0925	Long Pressed Name	Two Pointers, String
0929	Unique Email Addresses	Array, Hash Table, String
0942	DI String Match	Greedy, Array, Two Pointers +1
0944	Delete Columns to Make Sorted	Array, String

Medium (100)

Medium problems
No.	Problem	Topics
0003	Longest Substring Without Repeating Characters	Hash Table, String, Sliding Window
0005	Longest Palindromic Substring	Two Pointers, String, Dynamic Programming
0006	Zigzag Conversion	String
0008	String to Integer (atoi)	String
0012	Integer to Roman	Hash Table, Math, String
0017	Letter Combinations of a Phone Number	Hash Table, String, Backtracking
0022	Generate Parentheses	String, Dynamic Programming, Backtracking
0038	Count and Say	String
0043	Multiply Strings	Math, String, Simulation
0049	Group Anagrams	Array, Hash Table, String +1
0071	Simplify Path	Stack, String
0072	Edit Distance	String, Dynamic Programming
0079	Word Search	Depth-First Search, Array, String +2
0091	Decode Ways	String, Dynamic Programming
0093	Restore IP Addresses	String, Backtracking
0097	Interleaving String	String, Dynamic Programming
0131	Palindrome Partitioning	String, Dynamic Programming, Backtracking
0139	Word Break	Trie, Memoization, Array +3
0151	Reverse Words in a String	Two Pointers, String
0165	Compare Version Numbers	Two Pointers, String
0166	Fraction to Recurring Decimal	Hash Table, Math, String
0179	Largest Number	Greedy, Array, String +1
0187	Repeated DNA Sequences	Bit Manipulation, Hash Table, String +3
0208	Implement Trie (Prefix Tree)	Design, Trie, Hash Table +1
0211	Design Add and Search Words Data Structure	Depth-First Search, Design, Trie +1
0227	Basic Calculator II	Stack, Math, String
0241	Different Ways to Add Parentheses	Recursion, Memoization, Math +2
0299	Bulls and Cows	Hash Table, String, Counting
0306	Additive Number	String, Backtracking
0316	Remove Duplicate Letters	Stack, Greedy, String +1
0318	Maximum Product of Word Lengths	Bit Manipulation, Array, String
0331	Verify Preorder Serialization of a Binary Tree	Stack, Tree, String +1
0385	Mini Parser	Stack, Depth-First Search, String
0388	Longest Absolute File Path	Stack, Depth-First Search, String
0394	Decode String	Stack, Recursion, String
0395	Longest Substring with At Least K Repeating Characters	Hash Table, String, Divide and Conquer +1
0399	Evaluate Division	Depth-First Search, Breadth-First Search, Union Find +4
0402	Remove K Digits	Stack, Greedy, String +1
0423	Reconstruct Original Digits from English	Hash Table, Math, String
0424	Longest Repeating Character Replacement	Hash Table, String, Sliding Window
0433	Minimum Genetic Mutation	Breadth-First Search, Hash Table, String
0438	Find All Anagrams in a String	Hash Table, String, Sliding Window
0443	String Compression	Two Pointers, String
0449	Serialize and Deserialize BST	Tree, Depth-First Search, Breadth-First Search +4
0451	Sort Characters By Frequency	Hash Table, String, Bucket Sort +3
0467	Unique Substrings in Wraparound String	String, Dynamic Programming
0468	Validate IP Address	String
0474	Ones and Zeroes	Array, String, Dynamic Programming
0481	Magical String	Two Pointers, String
0516	Longest Palindromic Subsequence	String, Dynamic Programming
0522	Longest Uncommon Subsequence II	Array, Hash Table, Two Pointers +2
0524	Longest Word in Dictionary through Deleting	Array, Two Pointers, String +1
0535	Encode and Decode TinyURL	Design, Hash Table, String +1
0537	Complex Number Multiplication	Math, String, Simulation
0539	Minimum Time Difference	Array, Math, String +1
0556	Next Greater Element III	Math, Two Pointers, String
0567	Permutation in String	Hash Table, Two Pointers, String +1
0583	Delete Operation for Two Strings	String, Dynamic Programming
0592	Fraction Addition and Subtraction	Math, String, Simulation
0606	Construct String from Binary Tree	Tree, Depth-First Search, String +1
0609	Find Duplicate File in System	Array, Hash Table, String
0640	Solve the Equation	Math, String, Simulation
0647	Palindromic Substrings	Two Pointers, String, Dynamic Programming
0648	Replace Words	Trie, Array, Hash Table +1
0649	Dota2 Senate	Greedy, Queue, String
0676	Implement Magic Dictionary	Depth-First Search, Design, Trie +2
0677	Map Sum Pairs	Design, Trie, Hash Table +1
0678	Valid Parenthesis String	Stack, Greedy, String +1
0686	Repeated String Match	String, String Matching
0692	Top K Frequent Words	Trie, Array, Hash Table +5
0712	Minimum ASCII Delete Sum for Two Strings	String, Dynamic Programming
0720	Longest Word in Dictionary	Trie, Array, Hash Table +2
0721	Accounts Merge	Depth-First Search, Breadth-First Search, Union Find +4
0722	Remove Comments	Array, String
0752	Open the Lock	Breadth-First Search, Array, Hash Table +1
0756	Pyramid Transition Matrix	Bit Manipulation, Hash Table, String +1
0763	Partition Labels	Greedy, Hash Table, Two Pointers +1
0767	Reorganize String	Greedy, Hash Table, String +3
0777	Swap Adjacent in LR String	Two Pointers, String
0784	Letter Case Permutation	Bit Manipulation, String, Backtracking
0791	Custom Sort String	Hash Table, String, Sorting
0792	Number of Matching Subsequences	Trie, Array, Hash Table +4
0809	Expressive Words	Array, Two Pointers, String
0811	Subdomain Visit Count	Array, Hash Table, String +1
0816	Ambiguous Coordinates	String, Backtracking, Enumeration
0820	Short Encoding of Words	Trie, Array, Hash Table +1
0831	Masking Personal Information	String
0833	Find And Replace in String	Array, Hash Table, String +1
0838	Push Dominoes	Two Pointers, String, Dynamic Programming
0842	Split Array into Fibonacci Sequence	String, Backtracking
0848	Shifting Letters	Array, String, Prefix Sum
0856	Score of Parentheses	Stack, String
0880	Decoded String at Index	Stack, String
0890	Find and Replace Pattern	Array, Hash Table, String
0893	Groups of Special-Equivalent Strings	Array, Hash Table, String +1
0916	Word Subsets	Array, Hash Table, String
0921	Minimum Add to Make Parentheses Valid	Stack, Greedy, String
0926	Flip String to Monotone Increasing	String, Dynamic Programming
0937	Reorder Data in Log Files	Array, String, Sorting
0949	Largest Time for Given Digits	Array, String, Backtracking +1

Hard (45)

Hard problems
No.	Problem	Topics
0010	Regular Expression Matching	Recursion, String, Dynamic Programming
0030	Substring with Concatenation of All Words	Hash Table, String, Sliding Window
0032	Longest Valid Parentheses	Stack, String, Dynamic Programming
0044	Wildcard Matching	Greedy, Recursion, String +1
0065	Valid Number	String
0068	Text Justification	Array, String, Simulation
0076	Minimum Window Substring	Hash Table, String, Sliding Window
0087	Scramble String	String, Dynamic Programming
0115	Distinct Subsequences	String, Dynamic Programming
0126	Word Ladder II	Breadth-First Search, Hash Table, String +1
0127	Word Ladder	Breadth-First Search, Hash Table, String
0132	Palindrome Partitioning II	String, Dynamic Programming
0140	Word Break II	Trie, Memoization, Array +4
0212	Word Search II	Trie, Array, String +2
0214	Shortest Palindrome	String, String Matching, Hash Function +1
0224	Basic Calculator	Stack, Recursion, Math +1
0273	Integer to English Words	Recursion, Math, String
0282	Expression Add Operators	Math, String, Backtracking
0297	Serialize and Deserialize Binary Tree	Tree, Depth-First Search, Breadth-First Search +3
0301	Remove Invalid Parentheses	Breadth-First Search, String, Backtracking
0336	Palindrome Pairs	Trie, Array, Hash Table +1
0420	Strong Password Checker	Greedy, String, Heap (Priority Queue)
0466	Count The Repetitions	String, Dynamic Programming
0472	Concatenated Words	Depth-First Search, Trie, Array +3
0488	Zuma Game	Stack, Breadth-First Search, Memoization +2
0514	Freedom Trail	Depth-First Search, Breadth-First Search, String +1
0564	Find the Closest Palindrome	Math, String
0591	Tag Validator	Stack, String
0639	Decode Ways II	String, Dynamic Programming
0664	Strange Printer	String, Dynamic Programming
0691	Stickers to Spell Word	Bit Manipulation, Memoization, Array +5
0730	Count Different Palindromic Subsequences	String, Dynamic Programming
0736	Parse Lisp Expression	Stack, Recursion, Hash Table +1
0745	Prefix and Suffix Search	Design, Trie, Array +2
0761	Special Binary String	Recursion, String
0828	Count Unique Characters of All Substrings of a Given String	Hash Table, String, Dynamic Programming
0839	Similar String Groups	Depth-First Search, Breadth-First Search, Union Find +3
0854	K-Similar Strings	Breadth-First Search, Hash Table, String
0899	Orderly Queue	Math, String, Sorting
0902	Numbers At Most N Given Digit Set	Array, Math, String +2
0903	Valid Permutations for DI Sequence	String, Dynamic Programming, Prefix Sum
0906	Super Palindromes	Math, String, Enumeration
0936	Stamping The Sequence	Stack, Greedy, Queue +1
0940	Distinct Subsequences II	String, Dynamic Programming
0943	Find the Shortest Superstring	Bit Manipulation, Array, String +2

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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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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

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

String problems by difficulty

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Medium (100)100

Hard (45)45

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