leetcodeOJ 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]]

思路：回溯

代码如下：

class Solution {public:    vector<vector<int>> combinationSum3(int k, int n) {        vector<vector<int>> ans;        // if(k < 1 || n < 1)        //     return ans;        vector<int> re;        genComSum3(ans, re, 1, 9, k, n);        return ans;    }    private:    void genComSum3(vector<vector<int>>& ans, vector<int>& re, int start, int end, int k, int n){        if(n == 0 && k == 0){            ans.push_back(re);            return;        }        if(n < 0 || start > end)            return;        re.push_back(start);        genComSum3(ans, re, start+1, end, k-1, n-start);        re.pop_back();        genComSum3(ans, re, start+1, end, k, n);    }};