Problem

Given a singly linked list, write a function to check if the list is a palindrome. Return true if it is, false otherwise.
Input: 1->2->3->2->1
Output: true

Solution

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def isPalindrome(self, head: ListNode) -> bool:
        if not head:
            return True
        slow = head
        fast = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        if fast:
            slow = slow.next
        slow = self.reverse(slow)
        while slow:
            if slow.val != head.val:
                return False
            slow = slow.next
            head = head.next
        return True
    
    def reverse(self, head):
        prev = None
        while head:
            temp = head.next
            head.next = prev
            prev = head
            head = temp
        return prev