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：
回溯法。
void combinationSum3(vector<vector<int>>&ret, vector<int>&comb, int k, int n,int &sum, int begin) { if (sum > n)return; if (comb.size() == k && sum != n)return; else if (comb.size() == k && sum == n) { ret.push_back(comb); return; } for (int i = begin; i <= 9;i++) { comb.push_back(i); sum += i; combinationSum3(ret, comb, k, n, sum, i + 1); sum -= i; comb.pop_back(); } } vector<vector<int>> combinationSum3(int k, int n) { vector<vector<int>> ret; vector<int> comb; int sum = 0; combinationSum3(ret, comb, k, n, sum, 1); return ret; }
[leetcode-216-Combination Sum III]
原文：http://www.cnblogs.com/hellowooorld/p/7073441.html