链表问题——判断一个链表是否为回文结构

【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;    }}