Leetcode dfs Combinations

Combinations

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Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,

If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

题意：给定两个数字 n 和 k，给出从1...n数字中取 k个的所有可能组合

思路：dfs

递归的深度为 k，第 i 层的第j个节点有  n - i - j 个选择分支

复杂度：时间 O(n^k)，空间O(k)

vector<vector<int> > res;
int nn, kk;
void dfs(int cur, int start, vector<int> &path){
	if(cur == kk ){
		res.push_back(path);
	}
	for(int i = start; i <= nn; i++){
		path.push_back(i);
		dfs(cur + 1, i + 1, path);
		path.pop_back();
	}
}

vector<vector<int> > combine(int n, int k){
	nn = n, kk = k;
	vector<int> path;
	dfs(0, 1, path);
	return res;
}