链表问题——判断一个链表是否为回文结构

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【题目】
　　给定一个链表的头结点head，请判断该链表是否为回文结构
　　例如：
　　1->2->1 返回true
　　1->2->3 返回false

【代码】
　　演示三种方法
　　

import java.util.*;



class test{ 
    class Node{
        public Node head;
        public int value;
        public Node next;

        public Node(int data){
            this.value = data;
        }
    }
    public static boolean isPalindrom1(Node head){
        if(head==null||head.next==null){
            return true;
        }
        Stack<Node> stack = new Stack<Node>();
        Node cur = head;
        while(cur!=null){
            stack.push(cur);
            cur = cur.next;
        }
        while(head!=null){
            if(head.value != stack.pop().value){
                return false;
            }
            head = head.next;
        }
        return true;
    }

    public static boolean isPalindrom2(Node head){
        if(head==null||head.next==null){
            return true;
        }
        Node right = head.next;
        Node cur = head;
        while(cur.next!=null && cur.next.next!=null){
            right = right.next;
            cur = cur.next.next;
        }
        Stack<Node> stack = new Stack<Node>();
        while(right!=null){
            stack.push(right);
            right = right.next;
        }
        while(!stack.isEmpty()){
            if(head.value!=stack.pop().value){
                return false;
            }
            head = head.next;
        }
        return true;
    }

    public static boolean isPalindrom3(Node head){
        if(head==null || head.next==null){
            return true;
        }
        Node n1 = head;
        Node n2 = head;
        while(n2.next!=null&&n2.next.next!=null){
            n1 = n1.next;
            n2 = n2.next.next;
        }
        Node n3 = n1.next;
        n1.next = null;
        while(n3!=null){
            n2 = n3.next;
            n3.next = n1;
            n1 = n3;
            n3 = n2;
        }
        n3 = n1;
        while(head.next!=null){
            if(head.value!=n1.value){
                return false;
            }
            head = head.next;
            n1 = n1.next;
        }
        n1 = n3.next;
        while(n3!=null){
            n2 = n1.next;
            n1.next = n3;
            n1 = n2;
            n3 = n1.next;
        }
        return true;
    }
}