Combination Sum III

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Ensure that numbers within the set are sorted in ascending order.

Example 1:

Input: k = 3, n = 7

Output:

 

[[1,2,4]]

 

Example 2:

Input: k = 3, n = 9

Output:

 

[[1,2,6], [1,3,5], [2,3,4]]

思路：这道题和Combination Sum II的思路类似。只不过candidates数组里只有1-9这9个数，且增加了累加次数的限制。

class Solution {
public:
    void assist(vector<vector<int> >& res, vector<int>& cur, int target, int k, int st)
    {
        if (k == 0)
        {
            if (target == 0)
                res.push_back(cur);
            return;
        }
        for (int i = st; i <= 9; i++)
        {
            cur.push_back(i);
            assist(res, cur, target - i, k - 1, i + 1);
            cur.pop_back();
        }
    }
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<vector<int> > res;
        vector<int> cur;
        assist(res, cur, n, k, 1);
        return res;
    }
};