Reorder List

Given a singly linked list L: L0->L1->...->Ln-1->Ln, reorder it to: L0->Ln->L1->Ln-1->L2->Ln-2->... Tags: Linked List

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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 not head:
            return
        
        fast = head.next
        slow = head
        # ensure the first part has the same or one more node
        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
        
        # reverse the second half
        cur = slow.next
        slow.next = None
        prev = None
        while cur:
            tmp = cur.next
            cur.next = prev
            prev = cur
            cur = tmp
        
        # combine first part and second part
        first = head
        second = prev
        while second:
            tmp = second.next
            second.next = first.next
            first.next = second
            first = first.next.next
            second = tmp