77.组合

77.组合

思路：回溯

class Solution {
public:
    vector<int>tmp;
    vector<vector<int>>res;
    vector<vector<int>> combine(int n, int k) {
        backtrack(1,n,k);
        return res;
    }
    void backtrack(int start,int n,int k){
        if(tmp.size()==k){
            res.push_back(tmp);
            return;
        }
        for(int i=start;i<=n;i++){
            tmp.push_back(i);
            backtrack(i+1,n,k);
            tmp.pop_back();
        }
    }

};`