216.组合总和III

方法：回溯

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> ans = new ArrayList<>();
        Deque<Integer> path = new ArrayDeque<>(); 
        dfs(ans,path,n,k,1);
        return ans;
    }
    public void dfs(List<List<Integer>> ans,Deque<Integer> path,int target,int k,int begin){
        if(target == 0 && path.size() == k){
            ans.add(new ArrayList<Integer>(path));
            return;
        }
        for(int i = begin;i< 10;i++){
            if(target - i < 0) return;
            path.addLast(i);
            dfs(ans,path,target-i,k,i+1);
            path.removeLast();
        }
    }
}