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Leetcode234-palindromeLinkedList

Posted on Edited on In Views: Valine:
Solution Report of LeetCode Acceptted

Description

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

Example

Example 1:

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Input: 1->2
Output: false

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

Follow up:

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

Solution

  1. Move: fast pointer goes to the end, and slow goes to the middle.
  2. Reverse: the right half is reversed, and slow pointer becomes the 2nd head.
  3. Compare: run the two pointers head and slow together and compare.
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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;
        }
        if (fast != null) { // odd nodes: let right half smaller
            slow = slow.next;
        }
        slow = reverse(slow);
        fast = head;
    
        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;
    }
}