【算法与数据结构相关】【LeetCode】【234 回文链表】【Python】

题目：请判断一个链表是否为回文链表。你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？

示例：

输入: 1->2
输出: false
输入: 1->2->2->1
输出: true

思路：先遍历一遍链表得到链表长度，然后从链表中间开始将后半段反转，然后从两个链表头依次判断当前节点是否相同，若相同则挑战到下一个节点，不相同则返回false。

代码：

# 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
        """
        temp = head
        lenth = 0
        while temp:
            lenth += 1
            temp = temp.next
        if lenth <=1 :
            return True
        bhead = head
        for i in range(lenth//2):
            bhead = bhead.next
        if not bhead.next == None:
            pre = None
            cur = bhead
            nxt = bhead.next
            while not nxt == None:
                cur.next = pre
                pre = cur
                cur = nxt
                nxt = nxt.next
            cur.next = pre
            bhead = cur
        for i in range(lenth//2):
            if head.val == bhead.val:
                head = head.next
                bhead = bhead.next
            else:
                return False
        return True