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palindrome linked list.py
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palindrome linked list.py
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'''
Given a singly linked list, determine if it is a palindrome.
Follow up:
Could you do it in O(n) time and O(1) space?
'''
class Solution(object):
def isPalindrome(self, head):
"""
:type head: ListNode
:rtype: bool
"""
if not head or not head.next:
return True
slow = head
fast = head.next
while fast and fast.next: # find middle node
slow = slow.next
fast = fast.next.next
current = slow.next
slow.next = None
pre = None
while current: # reverse latter half list
temp = current
current = current.next
temp.next = pre
pre = temp
temp = head
while pre: # compare palindrome
if pre.val != temp.val:
return False
pre = pre.next
temp = temp.next
return True
'''
if O(n) space used, we can use stack
'''
class Solution(object):
def isPalindrome(self, head):
"""
:type head: ListNode
:rtype: bool
"""
if not head or not head.next:
return True
stack = [head.val]
slow = head
fast = head.next
while fast and fast.next:
slow = slow.next
stack.append(slow.val)
fast = fast.next.next
if not fast:
stack.pop()
temp = slow.next
while temp:
key = stack.pop()
if key != temp.val:
return False
temp = temp.next
return True