Leetcode backtracking method four board one solution template

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1 General backtracking template

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vector<xxx> solution;
void backtrack(路径, 选择列表){
    
    
	if (满足终止条件) {
    
    
		添加路径(当前解);
		return;
	}
	for (选择 : 选择列表) {
    
    
		if (notValid(选择)) continue;
		做选择;     // 在选择列表中移除<选择>
		backtrack(路径, 选择列表);
		撤销选择;   // 将<选择>恢复到选择列表中
	}
}

2 Backtracking method commonly used four axes + one solution (first index + inPath + sort adjacent deduplication + set non-adjacent deduplication)

By training the classic topics, the most commonly used technique in solving the problem of backtracking main 4types, depending on the nature when solving problems through a variety can be mechanized to solving mix which , for these four tips are detailed below

Four boards and one solution	Features	example
`inPath备忘录(i = 0)`	`避免重复相同的选择`	Avoid `[1,1,1]`or `[2,2,2]`appear
`first索引(i = first)`	Avoid repeating the same choice + `避免顺序不同，但组合相同的解`	Avoid `[1,2,3]`and `[1,3,2]`co-occur
`sort相邻去重!(input[i-1]==input[i])`	`输入有重复`Time, avoid repeating exactly the same solution	Avoid `[2,2,3]`and `[2,2,3]`co-occur
`set非相邻去重!set.count(input[i])`	`输入有重复`+ `不能sort`, avoid repeating exactly the same solution	Same as above
`bool backtrack(args) {}`	Only return to `1个解`use`if (backtrack()) return true;`	——

[Remarks]: Two solutions with different but identical paths[1, 2, 3, 2] can appear[2, 3]

The usage of the above four boards and one solution in different questions, the specific usage method can be directly clicked on the question link to view

Four axes	Applicable scene	topic
`inPath备忘录(i = 0)`	arrangement	46. Full arrangement ; 47. Full arrangement II
`first索引(i = first)`	Combination/subset/split	77. Combination ; 39. Combination sum ; 216. Combination sum III ; 78. Subset ; 131. Split palindrome ; 93. Restore IP address
`sort相邻去重!(input[i-1]==input[i])`	The input is repeated (assisting the above scenario)	90. Subset II ; 40. Combination Sum II ; 47. Full Permutation II
`set非相邻去重!set.count(input[i])`	The input is duplicated + cannot be sorted	491. Increasing Subsequence
`bool backtrack(args) {}`	Only returns`1个解`	37. Solve Sudoku ; 332. Rearrange the itinerary

Other methods	Applicable scene	topic
Backtracking template + multi-condition pruning	Board problem etc.	51. Queen N ; 37. Solving Sudoku ; 17. Letter combination of phone number
Backtracking template + new data structure	Graph theory	332. Re-arrange the itinerary (Euler circuit)

Appendix: List of Brush Questions for this Topic

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