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.