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 {
	List<Integer> list = new ArrayList<Integer>();
	List<List<Integer>> res = new ArrayList<>();
	public List<List<Integer>> combine(int n, int k) {
		if (n < k) {
			return res;
		}
		combine(n,k,0,1);
		return res;
	}

	private void combine(int n, int k, int len, int start){
    	if(len == k){
    		List<Integer> lin = new ArrayList<>(list);
    		res.add(lin);
    		return;
    	}
    	
    	for(int i=start;i<=n;i++){
    		list.add(i);
    		combine(n,k,len+1,i+1);
    		list.remove(Integer.valueOf(i));
    	}
    }
}

[LeetCode]Combinations