[算法] Backtracking algorithm

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• [ ] 691. Stickers to Spell Word
• [x] 🟥🌟489. Robot Room Cleaner
• [ ] 🟨🌟301. Remove Invalid Parentheses
  • [x] 20. Valid Parentheses [Stack]
  • [x] 🟨🌟1963. Minimum Number of Swaps to Make the String Balanced [Stack]
• [ ] 140. Word Break II
  • [ ] 139. Word Break
• [x] 🟨🌟17. Letter Combinations of a Phone Number
• [ ] 282. Expression Add Operators
• [x] ✅⭐78. Subsets [典型的Backtracking]
  • Method 1. 选/不选；
  • Method 2. 选哪一个元素
  • [x] 90. Subsets II
  • [x] 131. Palindrome Partitioning [选哪一个元素]
• [ ] 267. Palindrome Permutation II
  • [ ] 266. Palindrome Permutation
• [ ] 113. Path Sum II [Tree+Backtracking]
  • [ ] 112. Path Sum
• [x] 🟨🌟79. Word Search [Backtracking]
  • [ ] 🟨🌟 212. Word Search II [Trie, Backtracking, Optimization]
• [x] 🟩🌟 22. Generate Parentheses
  • [x] 20. Valid Parentheses
  • [x] 17. Letter Combinations of a Phone Number

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Combinations & Combination Sum

• [x] 77. Combinations
• [x] 39. Combination Sum
• [x] 40. Combination Sum II
• [x] 216. Combination Sum III
• [x] 377. Combination Sum IV
  • Dynamic Programming

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• [ ] 473. Matchsticks to Square

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子集型回溯

对于子集型回溯问题，当处理当前元素的时候，可分为 选/不选 两种处理方式。另一种解法就是当需要选择第 idx 个元素时，该选择哪一个。

• Method 1. 选/不选；
• Method 2. 选哪一个元素 (我比较常用的)

子集型回溯问题

• [x] 17. Letter Combinations of a Phone Number
• [x] ✅⭐78. Subsets [典型的Backtracking]
  • [x] 90. Subsets II
• [x] 131. Palindrome Partitioning [选哪一个元素]

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组合型回溯

• [x] 77. Combinations
• [x] 216. Combination Sum III
• [x] 22. Generate Parentheses

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排列型回溯

• [x] 46. Permutations
• [x] 47. Permutations II
• [ ] 51. N-Queens