dfs二刷20200711

class Solution {
    List<List<Integer>> res = new ArrayList();
    Stack<Integer> path = new Stack();

    public List<List<Integer>> combine(int n, int k) {
        //k表示: 到终点还需要几个数
        dfs(n, 1, k);
        return res;
    }

    public void dfs(int n, int u, int k){
        if(k == 0){
            res.add(new ArrayList(path));
            return;
        }

        for(int i = u; i <= n; i++){
            path.push(i);
            dfs(n, i+1, k-1);
            path.pop();
        }
    }
}