组合总和3 leetcode 216

组合总和3

解题思路：递归回溯

class Solution {
    public List<List<Integer>> result = new ArrayList<List<Integer>>();
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<Integer> list = new ArrayList<>();
        combinationSum3(1,k,n,list);
        return result;
    }
    public void combinationSum3(int start, int k, int n, List<Integer> list) {
        if(k==0) {
            if(n==0) {
                List<Integer> newList = (List)((ArrayList)list).clone();
                result.add(newList);
            }
            return;
        }
        for(int i=start;i<10;++i) {
            list.add(i);
            combinationSum3(i+1,k-1,n-i,list);
            list.remove((Integer)i);
        }
    }
}