组合

class Solution {
    
   private List<List<Integer>> res;

        public List<List<Integer>> combine(int n, int k) {

            res = new ArrayList<List<Integer>>();
            if (n<1 || k<1 || n<k) {
                return res;
            }
            List<Integer> selected = new ArrayList<Integer>();
            pickNext(1, n, k, selected);
            return res;
        }

        public void pickNext(int min, int max, int k, List<Integer> selected) {

            if (k==0) {
                res.add(new ArrayList<Integer>(selected));
                return;
            }

            for (int i=min; i<=max-k+1; i++) {
                selected.add(i);
                pickNext(i+1, max, k-1, selected);
                selected.remove(selected.size()-1);
            }
        }
}