# Program to convert linked list by alternating nodes from front and back in Python

PythonServer Side ProgrammingProgramming

Suppose we have a singly linked list, we have to rearrange it such that we take: the last node, and then the first node, and then the second last node, and then the second node, and so on.

So, if the input is like [1,2,3,4,5,6,7,8,9], then the output will be [9, 1, 8, 2, 7, 3, 6, 4, 5, ]

To solve this, we will follow these steps:

• c := node

• l := a new list

• while c is not-null, do

• insert value of c at the end of l

• c := next of c

• c := node

• while c is not null and l is non-empty, do

• value of c := value of last element from l and delete it

• c := next of c

• if c is null, then

• come out from the loop

• value of c := value of last element from l and delete it

• c := next of c

• return node

Let us see the following implementation to get better understanding:

## Example

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

def make_list(elements):
for element in elements[1:]:
while ptr.next:
ptr = ptr.next
ptr.next = ListNode(element)

print('[', end = "")
while ptr:
print(ptr.val, end = ", ")
ptr = ptr.next
print(']')

class Solution:
def solve(self, node):
c = node
l = []
while c:
l.append(c.val)
c = c.next

c = node
while c and l:
c.val = l.pop()
c = c.next
if c == None:
break
c.val = l.pop(0)
c = c.next
return node

ob = Solution()
print_list(ob.solve(head))
[1,2,3,4,5,6,7,8,9]
[9, 1, 8, 2, 7, 3, 6, 4, 5, ]