77. Combinations

Given two integers n and k, return all possible combinations ofk 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],]

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找出一个序列中全部的长度为k的子序列。使用dfs的方法，分支为取当前的数和不取当前的数。当长度为k时添加到结果集中。

代码：

class Solution{public:vector<vector<int>> combine(int n, int k) {vector<int> tmp;dfs(tmp, 1, n, k);return res;}private:vector<vector<int>> res;void dfs(vector<int> &tmp, int m, int n, int k){if(tmp.size() == k){res.push_back(tmp);return;}if(m > n){return;}tmp.push_back(m);dfs(tmp, m+1, n, k);tmp.pop_back();dfs(tmp, m+1, n, k);}};