leetcode [216]Combination Sum III

Posted 2021-11-02 xiaobaituyun

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]]

题目大意：

给定1到9这九个数字，从中选取k个数字，使得这k个数字之和等于n。这k个数字的选择不能重复。

解法：

采用dfs做：

java:

class Solution {
    List<List<Integer>>res;
    private void dfs(int k,int n,int index,List<Integer>tmpSum){
        if(n<0) return;
        if(k==0){
            if(n==0) res.add(new ArrayList<>(tmpSum));
            return;
        }
        for(int i=index;i<10;i++){
            tmpSum.add(i);
            dfs(k-1,n-i,i+1,tmpSum);
            tmpSum.remove(tmpSum.size()-1);
        }
    }

    public List<List<Integer>> combinationSum3(int k, int n) {
        res=new ArrayList();
        List<Integer>tmpSum=new ArrayList<>();
        dfs(k,n,1,tmpSum);
        return res;
    }
}