216. Combination Sum III**

216. Combination Sum III**

https://leetcode.com/problems/combination-sum-iii/

题目描述

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]]

C++ 实现 1

可以说思路和 39. Combination Sum** 以及 40. Combination Sum II** 一致了, 只是这里候选数据集变成了 [1, ..., 9], 以及限制了组合的个数为 k. 另外方面需要注意组合中的数字不能重复.

class Solution {
private:
    void dfs(int target, int start, int k, vector<int> &cur, vector<vector<int>> &res) {
        if (target < 0 || cur.size() > k) return;
        if (target == 0 && cur.size() == k) {
            res.push_back(cur);
            return;
        }
        for (int i = start; i < 10; ++ i) {
            cur.push_back(i);
            dfs(target - i, i + 1, k, cur, res);
            cur.pop_back();
        }
    }
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<vector<int>> res;
        vector<int> cur;
        dfs(n, 1, k, cur, res);
        return res;
    }
};