反转链表

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题目描述

输入一个链表，反转链表后，输出新链表的表头。

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public ListNode ReverseList(ListNode head) {
    if (head == null || head.next == null) {
        return head;
    }
    ListNode p = head;
    ListNode q = head.next;
    ListNode r;
    head.next = null;
    while (q != null) {
        r = q.next;
        q.next = p;
        p = q;
        q = r;
    }
    head = p;
    return head;
}