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],
]

思路： 回溯法 剪枝叶

public class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(n == 0 || k == 0 || n < k) {
            return result;
        }
        List<Integer> list = new ArrayList<Integer>();
        helper(result, list, 1, n, k);
        return result;
    }
    
    public void helper(List<List<Integer>> result, List<Integer> list, int i, int n , int k) {
        if(list.size() == k) {
            result.add(new ArrayList<Integer>(list));
            return;
        }
        for(int j = i; j <= n; j++) {
            list.add(j);
            helper(result, list,j + 1, n, k);
            list.remove(list.size() - 1);
        }
    }
}