77. Combinations

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

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

public class Solution {
    public List<List<Integer>> combine(int n, int k) {
      List<List<Integer>> res = new ArrayList<>();
      if(n == 0) return res;
      int[] nums = new int[n];
      for(int i = 1; i <= n ; i++){
          nums[i-1] = i;
      }
      List<Integer> mem = new ArrayList<>();
      dfs(res, mem, nums, k , 0);
      return res;
    }
    public void dfs(List<List<Integer>> res, List<Integer> mem, int[] nums, int k, int deep){
        if(mem.size() == k){
            res.add(new ArrayList<>(mem));
            return;
        }
        for(int i = deep; i < nums.length ; i++){
            mem.add(nums[i]);
            dfs(res, mem, nums, k , i+1);
            mem.remove(mem.size()-1);
        }
    }
}