String

Reverse a string in-place using the two-pointer technique, swapping characters from both ends

Longest Common Prefix

Find the longest common prefix among all strings.

Roman to Integer

Convert a Roman numeral string into its integer value.

Isomorphic Strings

Check whether two strings can be transformed using a one-to-one character mapping.

Encode and Decode Strings

Encode a list of strings into one string and decode it back safely.

Longest Substring Without Repeating Characters

Find the length of the longest substring that has no repeated characters.

Longest Repeating Character Replacement

Find the longest substring that can become all one character after at most k replacements.

Minimum Window Substring

Hard

Find the smallest substring of s that contains every character of t.

Permutation in String

Check whether s2 contains any permutation of s1.

Find All Anagrams in a String

Find every start index where an anagram of p appears in s.

Valid Anagram

Check whether two strings contain the same characters with the same frequencies.

Group Anagrams

Group strings that are anagrams of each other.

Maximum Number of Balloons

Count how many times the word balloon can be formed from text.

Valid Palindrome

Check whether a string is a palindrome after ignoring non-alphanumeric characters and case.

Valid Palindrome II

Check whether a string can become a palindrome after deleting at most one character.

Longest Palindromic Substring

Find the longest contiguous substring that is a palindrome.

Palindromic Substrings

Count all palindromic substrings in a string.

Count Binary Substrings

Count substrings with equal consecutive 0s and 1s.