leetcode -- 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 ArrayList<ArrayList<Integer>> combine(int n, int k) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(k == 0){
            return result;
        }
        
        ArrayList<Integer> com = new ArrayList<Integer>();
        int depth = 1;
        generate(result, com, depth, n, k);
        return result;
    }
    
    public void generate(ArrayList<ArrayList<Integer>> result, ArrayList<Integer> com,
            int depth, int n, int k){
                if(com.size() == k){
                    ArrayList<Integer> tmp = new ArrayList<Integer>();
                    tmp.addAll(com);
                    result.add(tmp);
                    return;
                }
                
                for(int i = depth; i <= n; i++){
                    com.add(i);
                    generate(result, com, i + 1, n, k);
                    com.remove(com.size() - 1);
                }
                
            }
}