1. How to sort numbers with 2 stacks. (no duplicate values)

public class Solution {
  public void sort(LinkedList<Integer> s1) {
    LinkedList<Integer> s2 = new LinkedList<Integer>();
    while (!s1.isEmpty()) {
      int temp = s1.pop();
      while (!s2.isEmpty() && s2.peek() > temp) {
        s1.push(s2.pop());
      }
      s2.push(temp);
    }
    while (!s2.isEmpty()) {
      s1.push(s2.pop());
    }
  }
}