77. 组合——回溯算法+剪枝

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

Accepted
27/27 cases passed (20 ms)
Your runtime beats 58.49 % of cpp submissions
Your memory usage beats 64.95 % of cpp submissions (9.8 MB)