【LeetCode 216】 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]]

思路

DFS，每个位置安排 1~9 的数字，如果当前位置安排了 x ，下一个位置从大于当前位置的数字开始安排。

代码

class Solution {
public:
    vector<vector<int>> combinationSum3(int k, int n) {
        vector<vector<int>> ans;
        vector<int> res;
        
        dfs(k, n, 1, 0, res, ans, 0);
        return ans;
    }
    
    void dfs(int k, int n, int index, int cur, vector<int>& res, vector<vector<int>>& ans, int cnt) {
        if (cur == n && cnt == k) {
            vector<int> tmp {res.begin(), res.end()};
            ans.push_back(tmp);
            return;
        }
        if (cur > n || cnt > k) return;
        
        for (int i=index; i<=9; ++i) {
            res.push_back(i);
            dfs(k, n, i+1, cur+i, res, ans, cnt+1);
            res.pop_back();
        }
        return;
    }
};