reorder-list

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Description

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:

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

Example 2:

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

Solution(Python)

# 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: Optional[ListNode]) -> None:
        """
        Do not return anything, modify head in-place instead.
        """
        # find the middle point
        slow = head
        fast = head

        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
        # detach into two lists
        node1 = head
        node2 = slow.next
        slow.next = None
        # reverse the second list
        prev = None
        curr = node2
        next_ = None
        while curr:
            next_ = curr.next
            curr.next = prev
            prev = curr
            curr = next_
        node2 = prev
        # alternatively attach the two lists
        curr = ListNode(0)

        while node1 or node2:

            if node1:
                curr.next = node1
                node1 = node1.next
                curr = curr.next
            if node2:
                curr.next = node2
                node2 = node2.next
                curr = curr.next
        head = curr.next
        # Time Complexity = O(n)
        # Space Complexity = O(1)