【链表】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 (null == head || null == head.next) {
            return head;
        }

        ListNode lastNode = head;
        ListNode currNode = head.next;
        ListNode preNode = head.next.next;

        while(null != preNode){
            currNode.next = lastNode;

            if (lastNode == head) {
                lastNode.next = null;
            }

            lastNode = currNode;
            currNode = preNode;
            preNode = preNode.next;
        } 
        
        currNode.next = lastNode;

        return currNode;
    
    }
}