LeetCode-in-All
234. Palindrome Linked List
Easy
Given the head of a singly linked list, return true if it is a palindrome.
Example 1:
Input: head = [1,2,2,1]
Output: true
Example 2:
Input: head = [1,2]
Output: false
Constraints:
- The number of nodes in the list is in the range
[1, 105]. 0 <= Node.val <= 9
Follow up: Could you do it in O(n) time and O(1) space?
Solution
/**
* Definition for a singly-linked list.
* class ListNode {
* public $val = 0;
* public $next = null;
* function __construct($val = 0, $next = null) {
* $this->val = $val;
* $this->next = $next;
* }
* }
*/
class Solution {
/**
* @param ListNode $head
* @return Boolean
*/
public function isPalindrome($head) {
$array = [];
while ($head) {
$array[] = $head->val;
$head = $head->next;
}
$cnt = count($array);
for ($x = 0; $x < $cnt / 2; $x++) {
$yKey = $cnt - 1 - $x;
if ($array[$x] !== $array[$yKey]) {
return false;
}
}
return true;
}
}