/*
 * @lc app=leetcode id=77 lang=java
 *
 * [77] Combinations
 */

// @lc code=start
class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> r = new ArrayList<List<Integer>>();
        List<Integer> rt = new ArrayList<Integer>();
        dfsIterate(r,rt, 1, n, k);
        return r;
    }

    private void dfsIterate(List<List<Integer>> r, List<Integer> list, int c, int n, int k) {
        if(list.size() == k) {
            r.add(shallowCopy(list));
            return;
        }
        if(c > n) {
            return;
        }
        for(int i = c; i<=n; i++) {
            List<Integer> p = shallowCopy(list);
            p.add(i);
            dfsIterate(r, p, i+1, n, k);
        }
    }

    private List<Integer> shallowCopy(List<Integer> list) {
        return new ArrayList<>(list);
    }
}
// @lc code=end