234. Palindrome Linked List

234. Palindrome Linked List

Leetcode link for this question

Discription:

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?

Analyze:

#Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None
#Generate a linked list by a list and return the head node
def gen_ListNode(li):
    if not li:
        return None
    head=ListNode(li.pop(0))
    tmp=head
    while li:
            tmp.next=ListNode(li.pop(0))
            tmp=tmp.next
    return head
#Print a linked list from head to tail
def show_List(head):
    if not head:
        print empty
    while head:
        print head.val,
        if head.next:
            print '→',
        head=head.next
    print 

Code 1:

# 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:
            return True
        le=0
        pre=ListNode('x')
        pre.next=head
        while head:
            le+=1
            head=head.next
        if le==1:
            return True
        lo=0
        flag=pre.next
        while lo<le/2-1 :
            old_flag=flag
            old_flne=flag.next
            old_flnene=flag.next.next

            flag.next=old_flnene
            old_flne.next=pre.next
            pre.next=old_flne

            lo+=1
        #print le,flag.val
        head=pre.next
        if le%2:
            flag=flag.next
        a=pre.next
        b=flag.next
        while b:
            if a.val!=b.val:
                return False
            a=a.next
            b=b.next
        return True


head=gen_ListNode([1,2,3,4,3,2,1])
show_List(head)
s=Solution()
s.isPalindrome(head)

1 → 2 → 3 → 4 → 3 → 2 → 1





True

Submission Result:

Status: Accepted
Runtime: 160 ms
Ranking: beats 35.55%