/**
 * 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 prev = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode nextTemp = curr.next; // 保存下一个节点
            curr.next = prev;              // 反转当前节点的指针
            prev = curr;                   // 移动prev指针
            curr = nextTemp;                // 移动curr指针
    }
        return prev; // 新的头节点，即原链表的尾节点,由于当前curr为null，所以prev就是新的头节点
    }
}