216. Combination Sum III ------------M. !!!!

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.
Note:
All numbers will be positive integers.
The solution set must not contain duplicate combinations.
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]]

A:

就是递归

class Solution {
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        return helper(k, 1, n);
    }
private:
    vector<vector<int>> helper(int k, int start, int n) {
        if (n <= 0 || start > 9 || k <= 0) {
            return {};
        }
        if (start == n && k == 1) {
            return {{start}};
        }
        vector<vector<int>> res = helper(k, start + 1, n);
        vector<vector<int>> res2 = helper(k - 1, start + 1, n - start);
        for (auto& line : res2) {
            line.push_back(start);
        }
        res.insert(res.end(), res2.begin(), res2.end());
        return res;
    }
};

Mistakes:
1: 一开始 没有注意限制条件  1---9的意思。
2:  很傻的是，第二遍。错了好几个地方。  主要是截止条件 没有考虑 {{start}}的情况。 反而是考虑了一堆{}的情况， 应该返回个{{}}。 哎。 我是SB