from pythonds import Stack
#function to change infix notation to postfix notation

def infixToPostfix(expression):

    #creating a dictionary name prec which compares the keys and checks for precedence associative rule
    prec = {}

    #give '*' and '/' a value of 3 and '+' and '-' value of 2 and '(' a value of 1

    prec["*"] = 3
    prec["/"] = 3
    prec["+"] = 2
    prec["-"] = 2
    prec["("] = 1

    #creating a stack name store
    store = Stack()

    #creating an empty list name
    postfix_list = []

    #tokenList splits each passed elements to check for comparison
    tokenList = expression.split()

    #check for the next item in the expression
    for counter in tokenList:
        #if the item existing is in the below list push to the list

        if counter in "ABCDEFGHIJKLMNOPQRSTUVWXYZ" or counter in "0123456789":
            postfix_list.append(counter)
        elif counter == '(':
            store.push(counter)
        elif counter == ')':
            topToken = store.pop()
            while topToken != '(':
                postfix_list.append(topToken)
                topToken = store.pop()
        else:
            while (not store.isEmpty()) and \
               (prec[store.peek()] >= prec[counter]):
                  postfix_list.append(store.pop())
            store.push(counter)

    while not store.isEmpty():
        postfix_list.append(store.pop())
    return " ".join(postfix_list)

print(infixToPostfix("A * B + C * D"))