palindrome linked list

Author: always-maap

234.PalindromeLinkedList.py

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+"""
+234. Palindrome Linked List
+Easy
+Linked List | Two Pointers | Stack | Recursion
+---
+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
+
+Follow up: Could you do it in O(n) time and O(1) space?
+"""
+
+from typing import Optional
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+# O(n) time | O(1) space
+class Solution:
+    def isPalindrome(self, head: Optional[ListNode]) -> bool:
+        # find middle
+        walker = runner = head
+        while runner and runner.next:
+            walker = walker.next
+            runner = runner.next.next
+
+        # reverse second half
+        rev = None
+        curr = walker
+        while curr != None:
+            nextTemp = curr.next
+            curr.next = rev
+            rev = curr
+            curr = nextTemp
+
+        # check two half
+        while rev and rev.val == head.val:
+            head = head.next
+            rev = rev.next
+
+        return not rev