77. Combinations

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> res=new ArrayList<List<Integer>>();
        combine(n, k, new ArrayList<Integer>(), res);
        return res;
    }
    private void combine(int num, int k, List<Integer> list, List<List<Integer>> res){
        if(k==0)
        {
            res.add(new ArrayList<Integer>(list));
            return;
        }
        if(num==0)
            return;
        combine(num-1,k,list,res);
        list.add(num);
        combine(num-1,k-1,list,res);
        list.remove(list.size()-1);
    }
}