# Backtracking Study Guide :::warning [< Return to Home Page](https://hackmd.io/@siansiansu/HknJJm0W0) ::: Combination Problems -------------------- Combination problems involve selecting a subset of elements from a given set, where the order doesn't matter. - 🟨 [17\. Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/) - 🟨 [39\. Combination Sum](https://leetcode.com/problems/combination-sum/) \[[Solution](https://hackmd.io/@siansiansu/B1UngPI7A)\] - 🟨 [40\. Combination Sum II](https://leetcode.com/problems/combination-sum-ii/) \[[Solution](https://hackmd.io/@siansiansu/BJQZPvLQC)\] - 🟨 [77\. Combinations](https://leetcode.com/problems/combinations/) \[[Solution](https://hackmd.io/@siansiansu/Sy2xIv8QA)\] - 🟨 [22\. Generate Parentheses](https://leetcode.com/problems/generate-parentheses/) Permutation Problems -------------------- Permutation problems involve arranging all the elements of a set into different sequences. - 🟨 [46\. Permutations](https://leetcode.com/problems/permutations/) \[[Solution](https://hackmd.io/@siansiansu/Sy4-zMo80)\] - 🟨 [47\. Permutations II](https://leetcode.com/problems/permutations-ii/) Subset Problems --------------- Subset problems involve finding all possible subsets of a given set. - 🟨 [78\. Subsets](https://leetcode.com/problems/subsets/) - 🟨 [90\. Subsets II](https://leetcode.com/problems/subsets-ii/) Graph and Matrix Search Problems -------------------------------- These problems involve searching through graphs or matrices using backtracking. - 🟥 [37\. Sudoku Solver](https://leetcode.com/problems/sudoku-solver/) - 🟨 [79\. Word Search](https://leetcode.com/problems/word-search/) \[[Solution](https://hackmd.io/@siansiansu/rkyWBMD80)\] - 🟥 [212\. Word Search II](https://leetcode.com/problems/word-search-ii/) \[[Solution](https://hackmd.io/@siansiansu/HJGtwwdLC)\] Chessboard Problems ------------------- Chessboard problems often involve placing pieces on a board while satisfying certain constraints. - 🟥 [51\. N-Queens](https://leetcode.com/problems/n-queens/) - 🟥 [52\. N-Queens II](https://leetcode.com/problems/n-queens-ii/) Problem Difficulty Legend ------------------------- - 🟩 Easy - 🟨 Medium - 🟧 Medium-Hard - 🟥 Hard - ⬛ Very Hard Additional Resources -------------------- - [Introduction to Backtracking (Video)](https://www.youtube.com/watch?v=DKCbsiDBN6c) - [Backtracking Algorithm (GeeksforGeeks)](https://www.geeksforgeeks.org/backtracking-algorithms/) - [Backtracking Patterns (LeetCode Discussion)](https://leetcode.com/discuss/general-discussion/680706/Article-on-Backtracking-Patterns)