面试题：反转链表

题目描述：输入一个链表，反转链表后，输出新链表的表头。（画图容易理解）

方法1：

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null || head.next == null ) return head;
        ListNode pre = head;
        ListNode p = head.next;
        pre.next = null;
        ListNode nxt;
        while(p!=null){
            nxt = p.next;
            p.next = pre;
            pre = p;
            p = nxt;
        }
        return pre;
    }
}

方法2：

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null || head.next == null ) return head;
        ListNode p1 = head;
        ListNode p2 = head.next;
        ListNode p3 = p2.next;
        p1.next = null;
        while(p3!=null){
            p2.next = p1;
            p1 = p2;
            p2 = p3;
            p3 = p3.next;
        }
        //最后一个p3=null时p2的next没有设置
        p2.next = p1;
        return p2;
    }
}