15 反转链表ReverseList

输入一个链表，反转链表后，输出新链表的表头。

import java.util.*;
/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public static ListNode ReverseList(ListNode head) {
        ListNode nextnode = null;
        ListNode prenode = null;
        while(head!=null){
            nextnode = head.next;
            head.next = prenode;
            prenode = head;
            head = nextnode;
        }
        return prenode;
    }
    public static void main(String [] args){
        Scanner sc = new Scanner(System.in);
        System.out.println("请依次输入链表");
        ListNode head = null;
        if(sc.hasNext())    head = new ListNode(sc.nextInt());
        ListNode temp = head;
        while(sc.hasNext()){
            temp = new ListNode(sc.nextInt());
            temp = temp.next;
        }
        temp.next = null;
        ReverseList(head);
    }
}

主要是三个指针在操控着反转的链表,A - -> B - -> C - -> D - -> E - -> F

16             nextnode = head.next;
17             head.next = prenode;
18             prenode = head;
19             head = nextnode;

head一开始指向A

nextnode指向了B   

A指向了prenode也就是空   A和B之间的链断开

head代表的A给了prenode,现在prenode就是A了,成功实现了A的反转出来

在将B给了head,重新循环就OK