[leedcode 216] Combination Sum III

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.

Ensure that numbers within the set are sorted in ascending order.

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]]

public class Solution {
    List<List<Integer>> res;
    List<Integer> seq;
    public List<List<Integer>> combinationSum3(int k, int n) {
        //dfs,for循环和递归叠加。
        //需要保存level值，因为结果要求是递增的，同时为了确保是k个数，每一次递归，k要减一
        res=new ArrayList<List<Integer>>();
        seq=new ArrayList<Integer>();
        dfs(k,n,1,0);
        return res;
    }
    public void dfs(int k,int n,int start,int sum){
        if(sum==n&&k==0){
            res.add(new ArrayList<Integer>(seq));
        }else if(sum>n||k<=0) return;
        for(int i=start;i<=9;i++){
            seq.add(i);
            dfs(k-1,n,i+1,sum+i);
            seq.remove(seq.size()-1);
        }
    }
}