CareerCup

Google for this question and you will get many links. Below is just simple plain implementation.

# your code goes here

def get_result(operator, operand1, operand2):
	operand1 = int(operand1)
	operand2 = int(operand2)
	
	if operator == str("*"):
		return operand1*operand2;
	if operator == str("+"):
		return operand1+operand2;
	if operator == str("-"):
		return operand1-operand2;	
	if operator == str("/"):
		return operand1/operand2;
	if operator == str("="):
		return operand1 == operand2;		
		
def evaluate(expr):
	stack = []
	operand_cound = 0
	operators = ['*', '+', '-', '/', '=']
	for i in expr[::-1]:
		print i
		operand1 = None
		operator = None
		if i not in operators:
			stack.append(i)
		elif i in operators:
			operand1 = stack.pop()
			operand2 = stack.pop()
			stack.append(get_result(i, operand1, operand2))
	return stack.pop()

print evaluate("-*+4325")

Both prefix and postfix can be implemented using a simple strategy using a stack(LIFO). We assume the
current prefix/postfix expression is present in a queue(FIFO)
A general algo is:

Till queue empty
  Dequeue one element from queue
    if element is a number/operand or an = expression
      push it into the stack
    else //element is an operand like + - * etc
      pop top two elements from stack
      perform the operation using the operand
      push result onto stack

      if(stack becomes empty before two elements can be popped)
	error! 

At this point, your queue will contain one element i.e. the answer

public static boolean isInteger(String token) {
        if (token == null)
            return false;
        try {
            Integer.parseInt(token);
        } catch (NumberFormatException e) {
            return false;
        }

        return true;
    }

    public static void main(String[] args) {
        Deque<Character> stack = new ArrayDeque<>();

        Scanner scanner = new Scanner(System.in);

        Integer num1 = null, num2 = null;
        String token;
        while (scanner.hasNext()) {
            token = scanner.next();

            if (!isInteger(token)) {
                stack.addFirst(token.charAt(0));
            }
            else {
                if (num1 == null) {
                    num1 = new Integer(Integer.parseInt(token));
                }
                else {
                    num2 = new Integer(Integer.parseInt(token));

                    switch (stack.removeFirst()) {
                        case '+':
                            num1 = num1 + num2;
                            break;
                        case '-':
                            num1 = num1 - num2;
                            break;
                        case '*':
                            num1 = num1 * num2;
                            break;
                        case '/':
                            num1 = num1 / num2;
                            break;
                        default:
                            System.out.println("Unsupported operation");
                            return;
                    }
                }
            }
        }

        System.out.println("Answer: " + num1);
    }