String LeetCode Problems: All 672, With Python Solutions

Every problem in this library that LeetCode tags String — 672 in total, 568 of them with a complete Python solution, a worked example and the time and space complexity of the approach.

672 problems
196 Easy
335 Medium
141 Hard

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)

#	Problem	Difficulty	Topics
13	Roman to Integer	Easy	Hash Table, Math, String
14	Longest Common Prefix	Easy	Trie, Array, String
20	Valid Parentheses	Easy	Stack, String
28	Find the Index of the First Occurrence in a String	Easy	Two Pointers, String, String Matching
58	Length of Last Word	Easy	String
67	Add Binary	Easy	Bit Manipulation, Math, String +1
125	Valid Palindrome	Easy	Two Pointers, String
168	Excel Sheet Column Title	Easy	Math, String
171	Excel Sheet Column Number	Easy	Math, String
205	Isomorphic Strings	Easy	Hash Table, String
242	Valid Anagram	Easy	Hash Table, String, Sorting
257	Binary Tree Paths	Easy	Tree, Depth-First Search, String +2
290	Word Pattern	Easy	Hash Table, String
344	Reverse String	Easy	Two Pointers, String
345	Reverse Vowels of a String	Easy	Two Pointers, String
383	Ransom Note	Easy	Hash Table, String, Counting
387	First Unique Character in a String	Easy	Queue, Hash Table, String +1
389	Find the Difference	Easy	Bit Manipulation, Hash Table, String +1
392	Is Subsequence	Easy	Two Pointers, String, Dynamic Programming
405	Convert a Number to Hexadecimal	Easy	Bit Manipulation, Math, String
409	Longest Palindrome	Easy	Greedy, Hash Table, String
412	Fizz Buzz	Easy	Math, String, Simulation
415	Add Strings	Easy	Math, String, Simulation
434	Number of Segments in a String	Easy	String
459	Repeated Substring Pattern	Easy	String, String Matching
482	License Key Formatting	Easy	String
500	Keyboard Row	Easy	Array, Hash Table, String
504	Base 7	Easy	Math, String
520	Detect Capital	Easy	String
521	Longest Uncommon Subsequence I	Easy	String
541	Reverse String II	Easy	Two Pointers, String
551	Student Attendance Record I	Easy	String
557	Reverse Words in a String III	Easy	Two Pointers, String
599	Minimum Index Sum of Two Lists	Easy	Array, Hash Table, String
657	Robot Return to Origin	Easy	String, Simulation
680	Valid Palindrome II	Easy	Greedy, Two Pointers, String
696	Count Binary Substrings	Easy	Two Pointers, String
709	To Lower Case	Easy	String
748	Shortest Completing Word	Easy	Array, Hash Table, String
771	Jewels and Stones	Easy	Hash Table, String
796	Rotate String	Easy	String, String Matching
804	Unique Morse Code Words	Easy	Array, Hash Table, String
806	Number of Lines To Write String	Easy	Array, String
819	Most Common Word	Easy	Array, Hash Table, String +1
821	Shortest Distance to a Character	Easy	Array, Two Pointers, String
824	Goat Latin	Easy	String
830	Positions of Large Groups	Easy	String
844	Backspace String Compare	Easy	Stack, Two Pointers, String +1
859	Buddy Strings	Easy	Hash Table, String
884	Uncommon Words from Two Sentences	Easy	Hash Table, String, Counting
917	Reverse Only Letters	Easy	Two Pointers, String
925	Long Pressed Name	Easy	Two Pointers, String
929	Unique Email Addresses	Easy	Array, Hash Table, String
942	DI String Match	Easy	Greedy, Array, Two Pointers +1
944	Delete Columns to Make Sorted	Easy	Array, String

Medium (100)

#	Problem	Difficulty	Topics
3	Longest Substring Without Repeating Characters	Medium	Hash Table, String, Sliding Window
5	Longest Palindromic Substring	Medium	Two Pointers, String, Dynamic Programming
6	Zigzag Conversion	Medium	String
8	String to Integer (atoi)	Medium	String
12	Integer to Roman	Medium	Hash Table, Math, String
17	Letter Combinations of a Phone Number	Medium	Hash Table, String, Backtracking
22	Generate Parentheses	Medium	String, Dynamic Programming, Backtracking
38	Count and Say	Medium	String
43	Multiply Strings	Medium	Math, String, Simulation
49	Group Anagrams	Medium	Array, Hash Table, String +1
71	Simplify Path	Medium	Stack, String
72	Edit Distance	Medium	String, Dynamic Programming
79	Word Search	Medium	Depth-First Search, Array, String +2
91	Decode Ways	Medium	String, Dynamic Programming
93	Restore IP Addresses	Medium	String, Backtracking
97	Interleaving String	Medium	String, Dynamic Programming
131	Palindrome Partitioning	Medium	String, Dynamic Programming, Backtracking
139	Word Break	Medium	Trie, Memoization, Array +3
151	Reverse Words in a String	Medium	Two Pointers, String
165	Compare Version Numbers	Medium	Two Pointers, String
166	Fraction to Recurring Decimal	Medium	Hash Table, Math, String
179	Largest Number	Medium	Greedy, Array, String +1
187	Repeated DNA Sequences	Medium	Bit Manipulation, Hash Table, String +3
208	Implement Trie (Prefix Tree)	Medium	Design, Trie, Hash Table +1
211	Design Add and Search Words Data Structure	Medium	Depth-First Search, Design, Trie +1
227	Basic Calculator II	Medium	Stack, Math, String
241	Different Ways to Add Parentheses	Medium	Recursion, Memoization, Math +2
299	Bulls and Cows	Medium	Hash Table, String, Counting
306	Additive Number	Medium	String, Backtracking
316	Remove Duplicate Letters	Medium	Stack, Greedy, String +1
318	Maximum Product of Word Lengths	Medium	Bit Manipulation, Array, String
331	Verify Preorder Serialization of a Binary Tree	Medium	Stack, Tree, String +1
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
395	Longest Substring with At Least K Repeating Characters	Medium	Hash Table, String, Divide and Conquer +1
399	Evaluate Division	Medium	Depth-First Search, Breadth-First Search, Union Find +4
402	Remove K Digits	Medium	Stack, Greedy, String +1
423	Reconstruct Original Digits from English	Medium	Hash Table, Math, String
424	Longest Repeating Character Replacement	Medium	Hash Table, String, Sliding Window
433	Minimum Genetic Mutation	Medium	Breadth-First Search, Hash Table, String
438	Find All Anagrams in a String	Medium	Hash Table, String, Sliding Window
443	String Compression	Medium	Two Pointers, String
449	Serialize and Deserialize BST	Medium	Tree, Depth-First Search, Breadth-First Search +4
451	Sort Characters By Frequency	Medium	Hash Table, String, Bucket Sort +3
467	Unique Substrings in Wraparound String	Medium	String, Dynamic Programming
468	Validate IP Address	Medium	String
474	Ones and Zeroes	Medium	Array, String, Dynamic Programming
481	Magical String	Medium	Two Pointers, String
516	Longest Palindromic Subsequence	Medium	String, Dynamic Programming
522	Longest Uncommon Subsequence II	Medium	Array, Hash Table, Two Pointers +2
524	Longest Word in Dictionary through Deleting	Medium	Array, Two Pointers, String +1
535	Encode and Decode TinyURL	Medium	Design, Hash Table, String +1
537	Complex Number Multiplication	Medium	Math, String, Simulation
539	Minimum Time Difference	Medium	Array, Math, String +1
556	Next Greater Element III	Medium	Math, Two Pointers, String
567	Permutation in String	Medium	Hash Table, Two Pointers, String +1
583	Delete Operation for Two Strings	Medium	String, Dynamic Programming
592	Fraction Addition and Subtraction	Medium	Math, String, Simulation
606	Construct String from Binary Tree	Medium	Tree, Depth-First Search, String +1
609	Find Duplicate File in System	Medium	Array, Hash Table, String
640	Solve the Equation	Medium	Math, String, Simulation
647	Palindromic Substrings	Medium	Two Pointers, String, Dynamic Programming
648	Replace Words	Medium	Trie, Array, Hash Table +1
649	Dota2 Senate	Medium	Greedy, Queue, String
676	Implement Magic Dictionary	Medium	Depth-First Search, Design, Trie +2
677	Map Sum Pairs	Medium	Design, Trie, Hash Table +1
678	Valid Parenthesis String	Medium	Stack, Greedy, String +1
686	Repeated String Match	Medium	String, String Matching
692	Top K Frequent Words	Medium	Trie, Array, Hash Table +5
712	Minimum ASCII Delete Sum for Two Strings	Medium	String, Dynamic Programming
720	Longest Word in Dictionary	Medium	Trie, Array, Hash Table +2
721	Accounts Merge	Medium	Depth-First Search, Breadth-First Search, Union Find +4
722	Remove Comments	Medium	Array, String
752	Open the Lock	Medium	Breadth-First Search, Array, Hash Table +1
756	Pyramid Transition Matrix	Medium	Bit Manipulation, Hash Table, String +1
763	Partition Labels	Medium	Greedy, Hash Table, Two Pointers +1
767	Reorganize String	Medium	Greedy, Hash Table, String +3
777	Swap Adjacent in LR String	Medium	Two Pointers, String
784	Letter Case Permutation	Medium	Bit Manipulation, String, Backtracking
791	Custom Sort String	Medium	Hash Table, String, Sorting
792	Number of Matching Subsequences	Medium	Trie, Array, Hash Table +4
809	Expressive Words	Medium	Array, Two Pointers, String
811	Subdomain Visit Count	Medium	Array, Hash Table, String +1
816	Ambiguous Coordinates	Medium	String, Backtracking, Enumeration
820	Short Encoding of Words	Medium	Trie, Array, Hash Table +1
831	Masking Personal Information	Medium	String
833	Find And Replace in String	Medium	Array, Hash Table, String +1
838	Push Dominoes	Medium	Two Pointers, String, Dynamic Programming
842	Split Array into Fibonacci Sequence	Medium	String, Backtracking
848	Shifting Letters	Medium	Array, String, Prefix Sum
856	Score of Parentheses	Medium	Stack, String
880	Decoded String at Index	Medium	Stack, String
890	Find and Replace Pattern	Medium	Array, Hash Table, String
893	Groups of Special-Equivalent Strings	Medium	Array, Hash Table, String +1
916	Word Subsets	Medium	Array, Hash Table, String
921	Minimum Add to Make Parentheses Valid	Medium	Stack, Greedy, String
926	Flip String to Monotone Increasing	Medium	String, Dynamic Programming
937	Reorder Data in Log Files	Medium	Array, String, Sorting
949	Largest Time for Given Digits	Medium	Array, String, Backtracking +1

Hard (45)

#	Problem	Difficulty	Topics
10	Regular Expression Matching	Hard	Recursion, String, Dynamic Programming
30	Substring with Concatenation of All Words	Hard	Hash Table, String, Sliding Window
32	Longest Valid Parentheses	Hard	Stack, String, Dynamic Programming
44	Wildcard Matching	Hard	Greedy, Recursion, String +1
65	Valid Number	Hard	String
68	Text Justification	Hard	Array, String, Simulation
76	Minimum Window Substring	Hard	Hash Table, String, Sliding Window
87	Scramble String	Hard	String, Dynamic Programming
115	Distinct Subsequences	Hard	String, Dynamic Programming
126	Word Ladder II	Hard	Breadth-First Search, Hash Table, String +1
127	Word Ladder	Hard	Breadth-First Search, Hash Table, String
132	Palindrome Partitioning II	Hard	String, Dynamic Programming
140	Word Break II	Hard	Trie, Memoization, Array +4
212	Word Search II	Hard	Trie, Array, String +2
214	Shortest Palindrome	Hard	String, String Matching, Hash Function +1
224	Basic Calculator	Hard	Stack, Recursion, Math +1
273	Integer to English Words	Hard	Recursion, Math, String
282	Expression Add Operators	Hard	Math, String, Backtracking
297	Serialize and Deserialize Binary Tree	Hard	Tree, Depth-First Search, Breadth-First Search +3
301	Remove Invalid Parentheses	Hard	Breadth-First Search, String, Backtracking
336	Palindrome Pairs	Hard	Trie, Array, Hash Table +1
420	Strong Password Checker	Hard	Greedy, String, Heap (Priority Queue)
466	Count The Repetitions	Hard	String, Dynamic Programming
472	Concatenated Words	Hard	Depth-First Search, Trie, Array +3
488	Zuma Game	Hard	Stack, Breadth-First Search, Memoization +2
514	Freedom Trail	Hard	Depth-First Search, Breadth-First Search, String +1
564	Find the Closest Palindrome	Hard	Math, String
591	Tag Validator	Hard	Stack, String
639	Decode Ways II	Hard	String, Dynamic Programming
664	Strange Printer	Hard	String, Dynamic Programming
691	Stickers to Spell Word	Hard	Bit Manipulation, Memoization, Array +5
730	Count Different Palindromic Subsequences	Hard	String, Dynamic Programming
736	Parse Lisp Expression	Hard	Stack, Recursion, Hash Table +1
745	Prefix and Suffix Search	Hard	Design, Trie, Array +2
761	Special Binary String	Hard	Recursion, String
828	Count Unique Characters of All Substrings of a Given String	Hard	Hash Table, String, Dynamic Programming
839	Similar String Groups	Hard	Depth-First Search, Breadth-First Search, Union Find +3
854	K-Similar Strings	Hard	Breadth-First Search, Hash Table, String
899	Orderly Queue	Hard	Math, String, Sorting
902	Numbers At Most N Given Digit Set	Hard	Array, Math, String +2
903	Valid Permutations for DI Sequence	Hard	String, Dynamic Programming, Prefix Sum
906	Super Palindromes	Hard	Math, String, Enumeration
936	Stamping The Sequence	Hard	Stack, Greedy, Queue +1
940	Distinct Subsequences II	Hard	String, Dynamic Programming
943	Find the Shortest Superstring	Hard	Bit Manipulation, Array, String +2

When the String problem arrives live

Stealth Interview is a desktop app for macOS and Windows. It reads the coding problem off your screen, returns a working solution with a step-by-step explanation and its time and space complexity, and transcribes what the interviewer is saying — while staying invisible to screen sharing.

Get Stealth Interview See pricing