给你单链表的头节点 head ，请你反转链表，并返回反转后的链表。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null;
        ListNode temp = head;
        ListNode next = head;
        while(temp != null){
            next = next.next;
            temp.next = pre;
            pre = temp;
            temp = next;
        }
        return pre;
    }
}