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234. Palindrome Linked List

Easy

Given the head of a singly linked list, return true if it is a palindrome.

Example 1:

Input: head = [1,2,2,1]

Output: true

Example 2:

Input: head = [1,2]

Output: false

Constraints:

• The number of nodes in the list is in the range [1, 105].
• 0 <= Node.val <= 9

Follow up: Could you do it in O(n) time and O(1) space?

Solution

require_relative '../../com_github_leetcode/list_node'

# Definition for singly-linked list.
# class ListNode
#     attr_accessor :val, :next
#     def initialize(val = 0, _next = nil)
#         @val = val
#         @next = _next
#     end
# end
# @param {ListNode} head
# @return {Boolean}
def is_palindrome2(head)
    # find mid, reverse second half of list, compare the nodes of 'two' lists
    slow = fast = head
    while fast && fast.next do 
        slow = slow.next
        fast = fast.next.next
    end
    mid_head = slow

    curr = mid_head
    prev = nil
    while curr do 
        next_node = curr.next 
        curr.next = prev
        prev = curr
        curr = next_node
    end
    prev

    curr = head
    mid_curr= prev
    while curr && mid_curr do 
        if curr.val != mid_curr.val
            return false
        end
        
        curr = curr.next
        mid_curr = mid_curr.next
    end

    true
end

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