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.

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]]

思路：回溯法

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