题解 | #反转链表#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @return ListNode类
     */
    public ListNode ReverseList (ListNode head) {
        // write code here
	//判断链头和链头的下一地址是否为空，如果为空，直接返回head指针
        if(head == null || head.next == null) return head;
	//定义pre和behind指针，指向链头的下一节点
        ListNode pre = head.next, behind = head.next;
	//把原先的链头的下一地址指向为null(因为原先的链头要变成链尾)
        head.next = null;
	//循环执行直至pre指针为空
        while(pre != null){
            behind = behind.next;
		//pre链节点的下一地址指向head
            pre.next = head;
            head = pre;

            pre = behind;
        }
        return head;
    }
}