private void validateInput(ListNode<Integer> list) {
    if (list == null) {
      throw new IllegalArgumentException("You can't pass a null list as parameter.");
    }
  }
Iterative algorithm to solve this problem. The key of this algorithm is to use two pointers to
go just through the fifty percent of the list. Using a fast pointer we can create an stack to
generate a reversed list and then check if both lists are equals or not. The complexity order
of this algorithm is the same than the previous one in time and space terms but the execution
time is faster because we are just checking half of the list.
src/main/java/com/github/pedrovgs/problem62/PalindromeList.java