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> tmp;
dfs(k, n,res, tmp, 1);
return res;
}
private:
void dfs(int k, int n, vector<vector<int> >&res, vector<int>&tmp, int cur){
if (tmp.size()>k|| n < 0) return;

if (tmp.size() == k&&n == 0){
res.push_back(tmp);
return;
}
for (int i = cur; i <= 9; ++i){
tmp.push_back(i);
dfs(k, n - i,res, tmp, i + 1);//i+1，不是cur+1
tmp.pop_back();
}
}
};