leetcode 77. Combinations(java)

题目描述：

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

代码:

class Solution {
    public static List> combine(int n, int k) {
        List> res=new ArrayList>();
        if(n());
        for(List list:res){
            list.add(n);
        }
        res.addAll(combine(n-1,k));
        return res;
    }
}

分析：