反转单向链表

ListNode* ReverseList(ListNode* head) {
  struct ListNode* prev = NULL;
  struct ListNode* next = NULL;

  while (head != NULL) {
    next = head->next;
    head->next = prev;
    prev = head;
    head = next;
  }

  return prev;
}

NULL     [1] --> [2] --> [3]
prev     head    next
NULL <-- [1]     [2] --> [3]
         prev    head    next
NULL <-- [1] <-- [2]     [3]
                 prev    head