$3.$ def merge$($stack$1$: ListStack, stack$2$: ListStack$)$ ListStack: merge$()$ function merges two sorted ListStacks into one. Test case: stack$1$: stack$2$: $134572689$

This function should return $123456789$

$(**$By the way we have to use this way to solve the problem$**)$:

class LinkedStack:

def $\_\_$init$\_\_($self$)$:

#underlying data structure is a linked list

self.stack $=$ LinkedList$()$

$"""$return the number of elements in this stack"""

def $\_\_$len$\_\_($self$)$:

return self.stack.$\_\_$len$\_\_()$

$"""$return True if empty, False otherwise"""

def is$\_$empty$($self$)$:

return self.stack.is$\_$empty$()$

$"""$return top element"""

def top$($self$)$:

return self.stack.first$()$

$"""$add e to the top of the stack"""

def push$($self$,$ e$)$:

self.stack.add$\_$first$($e$)$

$"""$remove and return the top element"""

def pop$($self$)$:

return self.stack.remove$\_$first$()$

$"""$return a string representation of this stack"""

def $\_\_$str$\_\_($self$)$:

return self.stack.$\_\_$str$\_\_()$

class ListStack:

def $\_\_$init$\_\_($self$)$:

#underlying data structure is a list

self.stack $=$ list$()$

$"""$return the number of elements in this stack"""

def $\_\_$len$\_\_($self$)$:

return len$($self$.$stack$)$

$"""$return True if empty, False otherwise"""

def is$\_$empty$($self$)$:

return len$($self$.$stack$)==0$

$"""$return top element"""

def top$($self$)$:

if self.is$\_$empty$()$:

return None

return self.stack$[-1]$

$"""$add e to the top of the stack"""

def push$($self$,$ e$)$:

self.stack.append$($e$)$

$"""$remove and return the top element"""

def pop$($self$)$:

if self.is$\_$empty$()$:

return None

return self.stack.pop$()$

$"""$return a string representation of this stack"""

def $\_\_$str$\_\_($self$)$:

display $=$ self.stack $+[]$

display.reverse$()$

return str$($display$)$

if $\_\_$name$\_\_==\text{'}\_\_$main$\_\_\text{'}$:

# create a linkedstack

s$1=$ LinkedStack$()$

# insert $1,2,3,4,5$

for e in $(1,2,3,4,5)$:

s$1.$push$($e$)$

print$($s$1)$

# then remove everything

while not s$1.$is$\_$empty$()$:

s$1.$pop$()$

print$($s$1)$

s$2=$ ListStack$()$

for e in $(1,2,3,4,5)$:

s$2.$push$($e$)$

print$($s$2)$

while not s$2.$is$\_$empty$()$:

s$2.$pop$()$

print$($s$2)$