206. 反转链表
     

/**
 * 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) {
        //双指针法：先定义两个指针pre和cur分别指向null和head,一个临时指针temp
        ListNode pre = null;
        ListNode cur = head;
        ListNode temp = null;

        while (cur != null) {//循环遍历
            //临时指针先存放cur.next,因为它等会会改变方向，避免丢失引用
            temp = cur.next;
            //改变指针指向，由指向后一个节点改为指向前一个节点
            cur.next = pre;
            //移动pre, cur向后移，继续后续的反转的过程
            pre = cur;
            cur = temp;
        }

        return pre;
        
    }
}