Combination Sum III

Find all possible combinations of k numbers that add up to a numbern, 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]]

class Solution {public:    void visit(int pos, int k, int n, int &sum,  vector<int> &buf, vector<vector<int> > &result)    {        if (k == 0)        {            if (sum == n)            {                result.push_back(buf);            }            return;        }        for (int i = pos; i <= 9; i++)        {               if (sum + i > n)            {                break;            }            buf.push_back(i);            sum += i;            visit(i+1, k-1, n, sum, buf, result);            buf.pop_back();            sum -= i;        }    }        vector<vector<int>> combinationSum3(int k, int n) {        vector<vector<int> > result;        vector<int> buf;        int sum = 0;        visit(1, k, n, sum, buf, result);        return result;    }};