Day15 剑指offer：反转链表

输入一个链表，反转链表后，输出链表的所有元素。

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null) return null;
        if(head.next == null) return head;
        ListNode res = head;
        ListNode nextHead = head.next;
        ListNode node;
        head.next = null;
        while(nextHead.next != null){
            node = nextHead.next;
            nextHead.next = res;
            res = nextHead;
            nextHead = node;
        }
        nextHead.next = res;
        res = nextHead;
        return res;
    }
}