【leetcode】Combination Sum III（middle）

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：

组合问题，递归，1-9 每个数字都分放入和不放入两种情况。得到满足条件的就压入答案。

class Solution {
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<int> partans;
        vector<vector<int>> ans;
        combination(1, k, n, partans, ans);
        return ans;
    }

    void combination(int curNum, int k, int sum, vector<int> &partans, vector<vector<int>> &ans)
    {
        if(curNum > 10 || sum < 0 || k < 0) return; //注意这里是 > 10, 否则数字9的答案无法压入
        if(sum == 0 && k == 0)
        {
            ans.push_back(partans);
        }
        else
        {
            partans.push_back(curNum);
            combination(curNum + 1, k - 1, sum - curNum, partans, ans);
            partans.pop_back();
            combination(curNum + 1, k, sum, partans, ans);
        }
    }
};