Combination Sum III（LeetCode）

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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]]

给定1~9的数，找出k个数的和为n的所有组合数

思路：还是递归+dfs，然后就是判断，当前组合size为k，和为n.

class Solution {
public:
	vector<vector<int>> combinationSum3(int k, int n) {
		vector<vector<int> > ans;
		vector<int> cur;
		dfs(k, n, 1, ans, cur);
		return ans;
	}
	void dfs(int k, int n,int start, vector<vector<int> >& ans, vector<int>& cur) {
		if (cur.size() > k || n < 0) return;
		if (cur.size() == k && n == 0) {
			ans.push_back(cur);
			return;
		}
		for (int i = start; i <= 9; i++) {
			cur.push_back(i);
			dfs(k, n - i, i + 1, ans, cur);
			cur.pop_back();
		}
	}
};