234. Palindrome Linked List

Given a singly linked list, determine if it is a palindrome.

 

 

public class Solution {

    public boolean isPalindrome(ListNode head) {

        ListNode fast = head;

        ListNode slow = head;

        while (fast != null && fast.next != null) {

            fast = fast.next.next;

            slow = slow.next; 

        }

        if (fast != null) {

            slow = slow.next; //如1234321，则slow指向第二个3，如果是12344321则slow指向第二个4

        }

        slow = reverse(slow);//把321翻转成123

        fast = head;//fast指向开头的1

        while (slow != null) {

            if (fast.val != slow.val) {

                return false;

            }

            fast = fast.next;

            slow = slow.next;

        }

        return true;

    }

 

    public ListNode reverse(ListNode head) {

        ListNode prev = null;

        while (head != null){

            ListNode next = head.next;     

            head.next = prev;

            prev = head;

            head = next;

        }

        return prev;

    }

}