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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;
}
};

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