leetcode-easy-listnode-234 Palindrome Linked List

mycode   89.42%

# 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
        """
        total = 0
        p1 = p2 = head
        while p1:
            total += 1
            p1 = p1.next
        if total < 2:
            return True          
        half = total // 2 - 1
        while half :
            p2 = p2.next
            half -= 1
        if total % 2 == 1:
            part_2 = p2.next.next
        else:
            part_2 = p2.next
        p2.next , last = None , None
        
        while part_2:
            new_head = part_2.next
            part_2.next = last
            last = part_2
            part_2 = new_head
       
        while head:
            if not last or head.val != last.val:
                return False
            head , last= head.next , last.next
        return True
        

 

参考

1、使用快慢指针，凡是用了额外空间

class Solution:
    def isPalindrome(self, head: ListNode) -> bool:        
        if not head or not head.next:
            return True

        new_list = []

        # 快慢指针法找链表的中点
        slow = fast = head
        while fast and fast.next:
            new_list.insert(0, slow.val)
            slow = slow.next
            fast = fast.next.next

        if fast: # 链表有奇数个节点
            slow = slow.next

        for val in new_list:
            if val != slow.val:
                return False
            slow = slow.next
        return True

 

2、使用快慢指针找重点，其他思路和我相同

# 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

        # 快慢指针法找链表的中点
        slow = fast = head
        while fast.next and fast.next.next:
            slow = slow.next
            fast = fast.next.next

        slow = slow.next # slow指向链表的后半段
        slow = self.reverseList(slow)

        while slow:
            if head.val != slow.val:
                return False
            slow = slow.next
            head = head.next
        return True

    def reverseList(self, head):
        new_head = None
        while head:
            p = head
            head = head.next
            p.next = new_head
            new_head = p
        return new_head