class StackQueue:
"""
Implementation of Queue using two Stacks.
"""
def __init__(self, limit=10):
self.limit = limit
self.stack_one = Stack(limit)
self.stack_two = Stack(limit)
def enqueue(self, item):
if self.isFull():
raise QueueOverflowError("Cannot insert item in a full queue.")
self.stack_one.push(item)
def _move_item_to_second_stack(self):
if self.stack_two.is_empty():
while not self.stack_one.is_empty():
self.stack_two.push(self.stack_one.pop())
def dequeue(self):
if self.isEmpty():
raise QueueUnderflowError(
"Cannot Remove an item from empty Queue.")
self._move_item_to_second_stack()
return self.stack_two.pop()
def peek(self):
self._move_item_to_second_stack()
return self.stack_two.peek()
def isEmpty(self):
return self.stack_one.is_empty() and self.stack_two.is_empty()
def isFull(self):
return (self.stack_one.size() + self.stack_two.size()) >= self.limit
def infix_to_postfix(infix_expr):
prec = {}
prec["*"] = 3
prec["/"] = 3
prec["+"] = 2
prec["-"] = 2
prec["("] = 1
op_stack = Stack()
postfix_list = []
token_list = infix_expr.split()
for token in token_list:
if token in "ABCDEFGHIJKLMNOPQRSTUWXYZ" or token in "0123456789":
postfix_list.append(token)
elif token == '(':
op_stack.push(token)
elif token == ')':
top_token = op_stack.pop()
while top_token != '(':
postfix_list.append(top_token)
top_token = op_stack.pop()
else:
while (not op_stack.is_empty()) and \
(prec[op_stack.peek()] >= prec[token]):
postfix_list.append(op_stack.pop())
op_stack.push(token)
while not op_stack.is_empty():
postfix_list.append(op_stack.pop())
return " ".join(postfix_list)