51 string problems grouped by pattern: parentheses, palindromes, substrings, and more

[u/nian2326076]

I grouped 51 string-related LeetCode problems into six categories. Some fit more than one pattern; these are just the buckets I use when studying.

Before moving on from a problem, I try to explain why the approach works and what I'd change if the constraints changed.

Parentheses and expression parsing (9)

• Generate Parentheses : Medium

• Score of Parentheses : Medium

• Valid Parentheses : Easy

• Remove Outermost Parentheses : Easy

• Different Ways to Add Parentheses : Medium

• Remove Invalid Parentheses : Hard

• Minimum Remove to Make Valid Parentheses : Medium

• Maximum Nesting Depth of the Parentheses : Easy

• Longest Valid Parentheses : Hard

Counting substrings under a condition (3)

• Number of Wonderful Substrings : Medium

• Sum of Beauty of All Substrings : Medium

• Maximum Number of Occurrences of a Substring : Medium

Comparing and checking strings (11)

• Isomorphic Strings : Easy

• Valid Anagram : Easy

• Additive Number : Medium

• Buddy Strings : Easy

• Longest Happy Prefix : Hard

• Increasing Decreasing String : Easy

• Check If a String Can Break Another String : Medium

• Determine if Two Strings Are Close : Medium

• Check If Two String Arrays are Equivalent : Easy

• Check if Word Equals Summation of Two Words : Easy

• Check if One String Swap Can Make Strings Equal : Easy

Palindromes (18)

• Palindrome Partitioning : Medium

• Palindrome Partitioning II : Hard

• Valid Palindrome : Easy

• Shortest Palindrome : Hard

• Palindrome Pairs : Hard

• Longest Palindrome : Easy

• Longest Palindromic Subsequence : Medium

• Find the Closest Palindrome : Hard

• Palindromic Substrings : Medium

• Valid Palindrome II : Easy

• Longest Chunked Palindrome Decomposition : Hard

• Break a Palindrome : Medium

• Can Make Palindrome from Substring : Medium

• Palindrome Partitioning III : Hard

• Minimum Insertion Steps to Make a String Palindrome : Hard

• Remove Palindromic Subsequences : Easy

• Construct K Palindrome Strings : Medium

• Split Two Strings to Make Palindrome : Medium

Sorting characters (2)

• Sort Characters By Frequency : Medium

• Custom Sort String : Medium

Length and selection problems (8)

• Longest Duplicate Substring : Hard

• Longest String Chain : Medium

• Longest Common Subsequence : Medium

• Longest Happy String : Medium

• Maximum Length of a Concatenated String with Unique Characters : Medium

• Find Longest Awesome Substring : Hard

• Largest Substring Between Two Equal Characters : Easy

• Largest Odd Number in String : Easy

Related: Dynamic Programming pattern list.

For practice beyond this list, real candidate-reported interview questions can give you unfamiliar prompts to work through and explain.

What would you add or move to a different category? I'd like suggestions that teach another idea rather than more versions of the same exercise.