reversing a linked list by swapping the data:

ListNode left = head;
ListNode right = tail;
for(int i=0; i<size()/2; i++) {
  Object tmp=left;
  left = right;
  right = tmp;
  left = left.next;
  right = right.prev;
}

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reversing a singly linked list by only relinking:

ListNode current = head;
ListNode previous = null;
while(current != null) {
  ListNode next = current.next;
  current.next = previous;
  previous = current;
  current = next;
}
head = previous;

==================================================

reversing a singly linked list by only relinking:

if (size() <= 1) return;
ListNode previous = head;
ListNode current = head.next;
while (current != null) {
  ListNode nextCurr = current.next;
  ListNode nextPrev = current;
  previous.prev = current;
  current.next = previous;
  previous = nextPrev;
  current = nextCurr;
}  
ListNode tmp = head;
head = tail;
tail = tmp;
head.prev = null;
tail.next = null;