链表206.反转链表

使用三个引用，不需要额外分配内存空间，直接撸

public ListNode reverseList(ListNode head) {
        ListNode h1 = head;
        if (head == null) {
            return null;
        }
<pre><code>    if (h1.next == null) {
        return h1;
    }

    ListNode h2 = h1.next;
    ListNode h3 = h2.next;
    h1.next = null;
    while (h2 != null) {
        h2.next = h1;
        h1 = h2;
        h2 = h3;
        if (h3 != null) h3 = h3.next;
    }
    return h1;
}