回溯 Backtracking
25 題(24 題以此為主技法)
🎯 何時用
列舉所有子集/排列/組合/棋盤放置,需要剪枝。
🧩 模板心法
choose → explore → un-choose;用 start 索引或 used 陣列避免重複;不合法早剪枝。
Medium 18 題
- #39Combination Sum
- #79Word Search
- #17Letter Combinations of a Phone Number
- #22Generate Parentheses
- #46Permutations
- #78Subsets
- #131Palindrome Partitioning
- #40Combination Sum II
- #47Permutations II
- #77Combinations
- #90Subsets II
- #93Restore IP Addresses
- #473Matchsticks to Square
- #494Target Sum
- #698Partition to K Equal Sum Subsets
- #1239Maximum Length of a Concatenated String With Unique Characters
- #1849Splitting a String Into Descending Consecutive Values
- #1980Find Unique Binary String