# Question

Given a singly linked list L: L0→L1→…→Ln-1→Ln, reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You may not modify the values in the list’s nodes, only nodes itself may be changed.

Example 1:

Given 1->2->3->4, reorder it to 1->4->2->3. Example 2:

Given 1->2->3->4->5, reorder it to 1->5->2->4->3.

# Solution

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
"""
:rtype: void Do not return anything, modify head in-place instead.
"""
if head is None or head.next is None:
return

stack = list()
count = 0
while node is not None:
stack.append(node)
count += 1
node = node.next

half = count // 2 if count % 2 == 1 else count // 2 - 1
for i in range(half):
next_node = node.next
insertion = stack.pop()
node.next = insertion
insertion.next = next_node
node = next_node

if count % 2 == 1:
node.next = None
else:
insertion = stack.pop()
node.next = insertion
insertion.next = None