Reverse a linked list

基本題，就是 while 把指針轉向即可，然後要學會用 dummy node (好用！)

居然寫錯了.... 多練習幾次吧！

public void reverseLinkedList(ListNode head) {
    if (head == null)
        return head;

    ListNode dummy = null;
    while (head != null) {
        ListNode next = head.next;
        head.next = dummy;
        dummy = head;
        head = next;
    }
    return dummy;
}