[Solved] You are given the head of a singly linked-list. The list can be represented as: L0 → L1 → … → Ln – 1 → Ln Reorder the list to be on the following form: L0 → Ln → L1 → Ln – 1 → L2 → Ln – 2 → …

Question

You are given the head of a singly linked-list. The list can be represented as:

L0 → L1 → … → Ln - 1 → Ln

Reorder the list to be on the following form:

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

You may not modify the values in the list’s nodes. Only nodes themselves may be changed.

Example 1:

Machine Learning Projects
Input: head = [1,2,3,4]
Output: [1,4,2,3]

Example 2:

Machine Learning Projects
Input: head = [1,2,3,4,5]
Output: [1,5,2,4,3]

Constraints:

  • The number of nodes in the list is in the range [1, 5 * 104].
  • 1 <= Node.val <= 1000

Python Solution

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reorderList(self, head: ListNode) -> None:
        """
        Do not return anything, modify head in-place instead.
        """
        if head == None:
            return []
        slow = head
        fast = head.next

        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next

        second_list = slow.next
        slow.next = None

        prev = None
        curr = second_list

        while curr:
            nxt = curr.next
            curr.next = prev
            prev = curr
            curr = nxt

        second_list = prev
        first_list = head

        while second_list:
            temp1 = first_list.next
            temp2 = second_list.next
            second_list.next = None
            first_list.next = second_list
            second_list.next = temp1
            first_list = first_list.next.next
            second_list = temp2

        return head
 
Abhishek Sharma
Abhishek Sharma

Started my Data Science journey in my 2nd year of college and since then continuously into it because of the magical powers of ML and continuously doing projects in almost every domain of AI like ML, DL, CV, NLP.

Articles: 521

Leave a Reply

Your email address will not be published. Required fields are marked *