LeetCode/234_Palindrome_Linked_List.py at main · AnmolPatel20/LeetCode · GitHub

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"""
Given the head of a singly linked list, return true if it is a palindrome or false otherwise.


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

"""

# 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: Optional[ListNode]) -> bool:
        if not head or not head.next:
            return True

        slow = head
        fast = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next

        prev = None
        cur = slow
        while cur:
            nxt = cur.next
            cur.next = prev
            prev = cur
            cur = nxt

        left = head
        right = prev

        while right:
            if left.val != right.val:
                return False
            else:
                left = left.next
                right = right.next
        return True