力扣【206】反转链表

题目：

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

题解：

import java.util.*;

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
    }
}

class Solution {
    public ListNode reverseList(ListNode head) {
        //边界条件
        if (head == null || head.next == null) {
            return head;
        }
        ListNode pre = null;
        ListNode after = null;
        while (head != null) {
            after = head.next;
            head.next = pre;
            pre = head;
            head = after;
        }
        return pre;
    }
}

public class Main {
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        ListNode head1 = new ListNode(2);
        ListNode head2 = new ListNode(3);
        ListNode head3 = new ListNode(4);
        ListNode head4 = new ListNode(5);

        head.next = head1;
        head1.next = head2;
        head2.next = head3;
        head3.next = head4;

        Solution s = new Solution();
        ListNode res = s.reverseList(head);
        while (res != null) {
            System.out.print(res.val + "->");
        }
    }
}