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 List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> list=new ArrayList<List<Integer>>();
        List<Integer> item=new ArrayList<Integer>();
        backTracking(n, k, 1, item, list);
        return list;
    }

    public void backTracking(int n, int k, int start, List<Integer> item, List<List<Integer>> list){
        if(item.size()==k){
            list.add(new ArrayList<Integer>(item));
            return;
        }
        for(int i=start; i<=n; i++){
            item.add(i);
            backTracking(n, k, i+1, item, list);
            item.remove(item.size()-1);
        }

    }
}