Backtracking

• 排列问题 Permutations。第 46 题，第 47 题。第 60 题，第 526 题，第 996 题。

• 组合问题 Combination。第 39 题，第 40 题，第 77 题，第 216 题。

• 排列和组合杂交问题。第 1079 题。

• N 皇后终极解法(二进制解法)。第 51 题，第 52 题。

• 数独问题。第 37 题。

• 四个方向搜索。第 79 题，第 212 题，第 980 题。

• 子集合问题。第 78 题，第 90 题。

• Trie。第 208 题，第 211 题。

• BFS 优化。第 126 题，第 127 题。

• DFS 模板。(只是一个例子，不对应任何题)

• BFS 模板。(只是一个例子，不对应任何题)

No.	Title	Solution	Difficulty	TimeComplexity	SpaceComplexity	Favorite	Acceptance
0017	Letter Combinations of a Phone Number		Medium	O(log n)	O(1)		48.8%
0022	Generate Parentheses		Medium	O(log n)	O(1)		64.9%
0037	Sudoku Solver		Hard	O(n^2)	O(n^2)	❤️	46.1%
0039	Combination Sum		Medium	O(n log n)	O(n)		58.9%
0040	Combination Sum II		Medium	O(n log n)	O(n)		49.9%
0046	Permutations		Medium	O(n)	O(n)	❤️	66.2%
0047	Permutations II		Medium	O(n^2)	O(n)	❤️	49.1%
0051	N-Queens		Hard	O(n!)	O(n)	❤️	49.2%
0052	N-Queens II		Hard	O(n!)	O(n)	❤️	59.8%
0060	Permutation Sequence		Hard	O(n log n)	O(1)		39.2%
0077	Combinations		Medium	O(n)	O(n)	❤️	57.1%
0078	Subsets		Medium	O(n^2)	O(n)	❤️	64.6%
0079	Word Search		Medium	O(n^2)	O(n^2)	❤️	36.6%
0089	Gray Code		Medium	O(n)	O(1)		50.1%
0090	Subsets II		Medium	O(n^2)	O(n)	❤️	48.6%
0093	Restore IP Addresses		Medium	O(n)	O(n)	❤️	37.3%
0126	Word Ladder II		Hard	O(n)	O(n^2)	❤️	23.5%
0131	Palindrome Partitioning		Medium	O(n)	O(n^2)	❤️	51.5%
0211	Design Add and Search Words Data Structure		Medium	O(n)	O(n)	❤️	39.9%
0212	Word Search II		Hard	O(n^2)	O(n^2)	❤️	36.7%
0216	Combination Sum III		Medium	O(n)	O(1)	❤️	60.0%
0306	Additive Number		Medium	O(n^2)	O(1)	❤️	29.6%
0357	Count Numbers with Unique Digits		Medium	O(1)	O(1)		48.8%
0401	Binary Watch		Easy	O(1)	O(1)		48.3%
0526	Beautiful Arrangement		Medium	O(n^2)	O(1)	❤️	61.7%
0784	Letter Case Permutation		Medium	O(n)	O(n)		66.3%
0842	Split Array into Fibonacci Sequence		Medium	O(n^2)	O(1)	❤️	36.7%
0980	Unique Paths III		Hard	O(n log n)	O(n)		77.2%
0996	Number of Squareful Arrays		Hard	O(n log n)	O(n)		48.0%
1079	Letter Tile Possibilities		Medium	O(n^2)	O(1)	❤️	75.8%
1641	Count Sorted Vowel Strings		Medium				77.3%
1655	Distribute Repeating Integers		Hard				40.5%
1659	Maximize Grid Happiness		Hard				35.3%
1681	Minimum Incompatibility		Hard				35.2%
1688	Count of Matches in Tournament		Easy				83.0%
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