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],
]

题意：给你n个数，组成k位的密码，列出所有可能。

思路：递归，动态往里添加

代码：

class Solution {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> res = new ArrayList<>();
        fun(res, new ArrayList<Integer>(), 0, n, k);
        return res;
    }

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

}