代码随想录：组合优化三

class Solution {
public:
    vector<int> target;
    vector<vector<int>> res;

    vector<vector<int>> combinationSum3(int k, int n) {
        bk(k, n, 1);
        return res;
    }

    void bk(int k, int n, int start) {
        if (target.size() == k) {
            if (s(target) == n)
                res.push_back(target);
            return;
        }
        for (int i = start; i <= 9 - (k - target.size()) + 1; i++) {
            target.push_back(i);
            if(s(target)>n){
                target.pop_back();
                break;
            }
            bk(k, n, i + 1);
            target.pop_back();
        }
    }

    int s(vector<int> target) {
        int sum = 0;
        for (int t : target) {
            sum += t;
        }
        return sum;
    }
};