234. Palindrome Linked List

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Total Accepted: 39482
Total Submissions: 145556
Difficulty: Easy

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

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

Java:

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isPalindrome(ListNode head) {
        int len = 0;
        ListNode p = head, tmp, newHead = null;
        while (p != null) {
            p = p.next;
            ++len;
        }
        p = head;
        int halfLen = len >>> 1;
        for (int i = 0; i < halfLen; ++i) {
            tmp = p.next;
            p.next = newHead;
            newHead = p;
            p = tmp;
        }
        if (len % 2 == 1) {
            p = p.next;
        }
        for (int i = 0; i < halfLen; ++i) {
            if (newHead.val != p.val)
                return false;
            newHead = newHead.next;
            p = p.next;
        }
        return true;
    }
}

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