回溯法是一種系統性地搜尋所有可能解的演算法。透過「選擇、探索、撤銷」三步驟，在搜尋樹上進行深度優先搜尋。
Notes:
- 回溯的核心模板：做選擇 → 遞迴 → 撤銷選擇
- 剪枝 (Pruning) 是提升效率的關鍵
- 排列 vs 組合 vs 子集，三種變體的差異要掌握
跨倉庫導讀#
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Letter Combinations of a Phone Number
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Combination Sum
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Combination Sum II
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Permutations
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Permutations II
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N-Queens
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N Queens II
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Combinations
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Subsets
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Word Search
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Subsets II
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