Leetcode——216. Combination Sum III

题目原址

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

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.

Example1：

Input: k = 3, n = 7
Output: [[1,2,4]]

Example2：

Input: k = 3, n = 9
Output: [[1,2,6], [1,3,5], [2,3,4]]

解题思路

该提使用dfs的方法解决就可以了，属于DFS的一个基础应用

AC代码

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<Integer> list = new ArrayList<Integer>();
        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        re(ret, list, k, n, 0, 1);
        return ret;
    }

    public void re(List<List<Integer>> ret, List<Integer> list, int k, int n, int sum, int level) {
        if(sum == n && k == 0)
            ret.add(new ArrayList(list));
        else if(sum > n || k <= 0) return;

        for(int i = level; i <= 9; i++) {
            list.add(i);
            re(ret, list, k - 1, n, sum + i, i + 1);
            list.remove(list.size() - 1);
        }
    }
}