234.Palindrome Linked List

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Linked List, Easy

Question

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

Example 1:

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Input: 1->2
Output: false

Example 2:

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Input: 1->2->2->1
Output: true

Answer

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        ListNode fast = head, slow = head;
        while(fast!=null&&fast.next!=null){
            fast = fast.next.next;
            slow = slow.next;
        }
        //slow -- mid
        ListNode tail = reverse(slow);
        //compare head->mid, tail->mid if they are the same
        while(tail!=null&&head!=null){
            if(tail.val!=head.val)
                return false;
            tail = tail.next;
            head = head.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;
        }
        //worth mentioning: return prev, when head comes to the end, 'prev=head' prev will get the last node, head will be null
        return prev;
    }
}

Running time:O(n)