def infix_to_postfix(expr):
"""
infix convert to postfix
:param expr: infix expression
:return: postfix expression
Example:
>>> infix_to_postfix('(31+21)*33-31/17')
31 21 + 33 * 31 17 / -
"""
expr = spit_num(expr)
stack = Stack()
result_list = []
for item in expr:
if is_num(item):
result_list.append(item)
elif is_operator(item):
while True:
if stack.is_empty() or stack.peek() == '(' or cmp_priority(
item, stack.peek()):
stack.push(item)
break
result_list.append(stack.pop())
elif item == '(':
stack.push(item)
elif item == ')':
while stack.peek() != '(':
result_list.append(stack.pop())
stack.pop()
else:
raise ValueError
while not stack.is_empty():
result_list.append(stack.pop())
return ' '.join(result_list)
def infix_to_prefix(expr):
expr = spit_num(expr)[::-1]
stack = Stack()
result_list = []
for item in expr:
if is_num(item):
result_list.append(item)
elif item == ')':
stack.push(item)
elif is_operator(item):
if stack.is_empty() or stack.peek() == ')' or cmp_priority(
item, stack.peek()) > 0:
stack.push(item)
else:
while True:
result_list.append(stack.pop())
if stack.is_empty() or stack.peek() == ')' or cmp_priority(
item, stack.peek()) > 0:
stack.push(item)
break
else:
while stack.peek() != ')':
result_list.append(stack.pop())
stack.pop()
while not stack.is_empty():
result_list.append(stack.pop())
return ' '.join(result_list[::-1])