Reverse Linked List

 1 public static ListNode reverse(ListNode head) {
 2         if (head == null || head.next == null) return head;
 3         
 4         ListNode prev = null;
 5         ListNode curr = head;
 6         ListNode next;
 7         
 8         while (curr != null) {
 9             next = curr.next;
10             curr.next = prev;
11             prev = curr;
12             curr = next;
13         }
14         head = prev;
15         return head;
16     }