Palindrome Linked List

题目：

Implement a function to check if a linked list is a palindrome.

分析：

无

解法：

public class Solution {
    /**
     * @param head a ListNode
     * @return a boolean
     */
    public ListNode reverseList(ListNode head) {
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode prev = dummy;
        while (head.next != null) {
            ListNode newhead = head.next;
            head.next = newhead.next;
            newhead.next = prev.next;
            prev.next = newhead;
        }
        return dummy.next;
    } 

    public boolean isPalindrome(ListNode head) {
        // Write your code here
        if (head == null || head.next == null) {
            return true;
        }
        if (head.next.next == null) {
            return head.val == head.next.val;
        }
        ListNode slow = head;
        ListNode fast = head;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }

        ListNode head2 = slow.next;
        slow.next = null;
        head2 = reverseList(head2);

        while (head != null && head2 != null) {
            if (head.val != head2.val) {
                return false;
            } else {
                head = head.next;
                head2 = head2.next;
            }
        }
        return true;
    }
}