学习笔记 | Python版本 LeetCode 234. 回文链表

LeetCode 234. 回文链表

请判断一个链表是否为回文链表。

示例 1:

输入: 1->2
输出: false

示例 2:

输入: 1->2->2->1
输出: true

进阶：

• 你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def isPalindrome(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        if not head or not head.next:
            return True
            
        # Find the end of first half and reverse second half.
        
        first_half_end = self.end_of_first_half(head)
        second_half_start = self.reverse_list(first_half_end.next)

        # Check whether or not there's a palindrome.
        result = True
        first_position = head
        second_position = second_half_start

        while result and second_position:
            if first_position.val != second_position.val:
                result = False
            first_position = first_position.next
            second_position = second_position.next

        # Restore the list and return the result.
        first_half_end.next = self.reverse_list(second_half_start)
        return result    

    def end_of_first_half(self, head):
        fast = head
        slow = head
        while fast.next and fast.next.next:
            fast = fast.next.next
            slow = slow.next
        return slow

    def reverse_list(self, head):
        previous = None
        current = head
        while current:
            next_node = current.next
            current.next = previous
            previous = current
            current = next_node
        return previous