143. Reorder List

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:

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

Example 2:

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

如果單純list，穿插就想到zip，反向就想到[::-1]，但這題是linkedlist，可以用快慢指針。

# 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 []
        
        # Find the middle of the list
        slow = fast = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        
        slow.next,mid = None,slow.next # del last, mid=last node
        
        # Reverse the half after middle
        dummyHead = ListNode(0)
        while mid:
            print(mid.val)
            dummyHead.next, mid.next, mid = mid, dummyHead.next, mid.next 
            
        # Start reorder one by one  
        reverse = dummyHead.next
        while reverse:
            head.next,reverse.next,reverse,head = reverse,head.next,reverse.next,head.next

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