lintcode-medium-Palindrome Linked List

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

 

Example

Given 1->2->1, return true

Challenge

Could you do it in O(n) time and O(1) space?

 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    /**
     * @param head a ListNode
     * @return a boolean
     */
    public boolean isPalindrome(ListNode head) {
        // Write your code here
        
        if(head == null || head.next == null)
            return true;
        
        ListNode slow = head;
        ListNode fast = head;
        
        while(fast != null && fast.next != null && fast.next.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        
        ListNode head2 = slow.next;
        slow.next = null;
        
        head2 = reverse(head2);
        
        ListNode p1 = head;
        ListNode p2 = head2;
        
        while(p2 != null){
            if(p1.val != p2.val)
                return false;
            p1 = p1.next;
            p2 = p2.next;
        }
        
        return true;
    }
    
    public ListNode reverse(ListNode head){
        if(head == null || head.next == null)
            return head;
        
        ListNode prev = head;
        ListNode curr = head.next;
        ListNode next = head.next.next;
        
        head.next = null;
        
        while(next != null){
            curr.next = prev;
            prev = curr;
            curr = next;
            next = next.next;
        }
        curr.next = prev;
        
        return curr;
    }
    
}