77. 组合

输入: n = 4, k = 2
输出:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

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

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