leetcode77. 组合

class Solution {
public:
    vector<vector<int>>ans;
    vector<int>path;
    void dfs(int u,int n,int k){
        if(path.size()+n-u+1<k)return;//剪枝
        if(path.size()==k){
            ans.push_back(path);
            return ;
        }
        for(int i=u;i<=n;i++){
            path.push_back(i);
            dfs(i+1,n,k);
            path.pop_back();
        }
    }
    vector<vector<int>> combine(int n, int k) {
        dfs(1,n,k);
        return ans;
    }
};