[Leetcode] 0143. Reorder List

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內容目錄

Medium

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

L₀ → L₁ → … → L_{n - 1} → L_n

Reorder the list to be on the following form:

L₀ → L_n → L₁ → L_{n - 1} → L₂ → L_{n - 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

Python

				
					# time complexity: O(n)
# space complexity: O(1)
from typing import Optional


class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next


class Solution:
    def reorderList(self, head: Optional[ListNode]) -> None:
        if not head:
            return
        fast = head
        slow = head
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next

        prev = None
        curr = slow
        while curr:
            curr.next, prev, curr = prev, curr, curr.next

        first = head
        second = prev
        while second.next:
            first.next, first = second, first.next
            second.next, second = first, second.next

        return


root = ListNode(1)
root.next = ListNode(2)
root.next.next = ListNode(3)
root.next.next.next = ListNode(4)
root.next.next.next.next = ListNode(5)
root.next.next.next.next.next = ListNode(6)
print(Solution().reorderList(root))

Medium

Python

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