老师下面的代码是leetcode上给出的solution，我不懂的地方是在backtrack函数中，当我们找到一个solution时，会进行output.add(new LinkedList(curr))，为什么这里add的时候是new LinkedList，而不是new

class Solution {
  List<List<Integer>> output = new LinkedList();
  int n;
  int k;

  public void backtrack(int first, LinkedList<Integer> curr) {
    // if the combination is done
    if (curr.size() == k)
      output.add(new LinkedList(curr));

    for (int i = first; i < n + 1; ++i) {
      // add i into the current combination
      curr.add(i);
      // use next integers to complete the combination
      backtrack(i + 1, curr);
      // backtrack
      curr.removeLast();
    }
  }

  public List<List<Integer>> combine(int n, int k) {
    this.n = n;
    this.k = k;
    backtrack(1, new LinkedList<Integer>());
    return output;
  }
}