Palindrome Linked List

Author: volodimyr

234.palindrome_linked_list/palindrome.py

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+# 234. Palindrome Linked List
+# Topics: 'Stack', 'Recursion', 'Linked List', 'Two Pointers'
+# Given the head of a singly linked list, return true if it is a 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
+
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+# Memory O(n), Time O(n)
+class Solution:
+    def isPalindrome(self, head: Optional[ListNode]) -> bool:
+        arr = []
+        cur = head
+        while cur:
+            arr.append(cur.val)
+            cur = cur.next
+        
+        L, R = 0, len(arr)-1
+        while L < R:
+            if arr[L] != arr[R]:
+                return False
+            L+=1
+            R-=1
+        return True