LeetCode[216] 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.

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]]

class Solution {
public:
vector<vector<int>> combinationSum3(int k, int n) {
vector<vector<int>> res;
vector<int> vec;
combinationSum(res, vec, n, 1, k);
return res;
}
void combinationSum(vector<vector<int>>& res, vector<int>& vec, int sum, int start, int k)
{
if (sum < 0)
return;
if (sum == 0 && vec.size() == k)
res.push_back(vec);
else
{
for (int i = start; i <= 9; ++i)
{
vec.push_back(i);
combinationSum(res, vec, sum - i, i + 1, k);
vec.pop_back();
}
}
}
};