Morgan Stanley coding round questions 2017

Question:


Evaluate an expression represented by a String. Expression can contain parentheses, you can assume parentheses are well-matched. For simplicity, you can assume only binary operations allowed are +, -, *, and /. Arithmetic Expressions can be written in one of three forms:

In other way evaluate "5+2 ". Result should be displayed as 7

Infix Notation: Operators are written between the operands they operate on, e.g. 3 + 4 .

Prefix Notation: Operators are written before the operands, e.g + 3 4

Postfix Notation: Operators are written after operands

Algorithm:

1. While there are still tokens to be read in,
   1.1 Get the next token.
   1.2 If the token is:
       1.2.1 A number: push it onto the value stack.
       1.2.2 A variable: get its value, and push onto the value stack.
       1.2.3 A left parenthesis: push it onto the operator stack.
       1.2.4 A right parenthesis:
         1 While the thing on top of the operator stack is not a 
           left parenthesis,
             1 Pop the operator from the operator stack.
             2 Pop the value stack twice, getting two operands.
             3 Apply the operator to the operands, in the correct order.
             4 Push the result onto the value stack.
         2 Pop the left parenthesis from the operator stack, and discard it.
       1.2.5 An operator (call it thisOp):
         1 While the operator stack is not empty, and the top thing on the
           operator stack has the same or greater precedence as thisOp,
           1 Pop the operator from the operator stack.
           2 Pop the value stack twice, getting two operands.
           3 Apply the operator to the operands, in the correct order.
           4 Push the result onto the value stack.
         2 Push thisOp onto the operator stack.
2. While the operator stack is not empty,
    1 Pop the operator from the operator stack.
    2 Pop the value stack twice, getting two operands.
    3 Apply the operator to the operands, in the correct order.
    4 Push the result onto the value stack.
3. At this point the operator stack should be empty, and the value
   stack should have only one value in it, which is the final result.

Code:

import java.util.Stack;

 

public class EvaluateString

{

    public static int evaluate(String expression)

    {

        char[] tokens = expression.toCharArray();

 

         // Stack for numbers: 'values'

        Stack<Integer> values = new Stack<Integer>();

 

        // Stack for Operators: 'ops'

        Stack<Character> ops = new Stack<Character>();

 

        for (int i = 0; i < tokens.length; i++)

        {

             // Current token is a whitespace, skip it

            if (tokens[i] == ' ')

                continue;

 

            // Current token is a number, push it to stack for numbers

            if (tokens[i] >= '0' && tokens[i] <= '9')

            {

                StringBuffer sbuf = new StringBuffer();

                // There may be more than one digits in number

                while (i < tokens.length && tokens[i] >= '0' && tokens[i] <= '9')

                    sbuf.append(tokens[i++]);

                values.push(Integer.parseInt(sbuf.toString()));

            }

 

            // Current token is an opening brace, push it to 'ops'

            else if (tokens[i] == '(')

                ops.push(tokens[i]);

 

            // Closing brace encountered, solve entire brace

            else if (tokens[i] == ')')

            {

                while (ops.peek() != '(')

                  values.push(applyOp(ops.pop(), values.pop(), values.pop()));

                ops.pop();

            }

 

            // Current token is an operator.

            else if (tokens[i] == '+' || tokens[i] == '-' ||

                     tokens[i] == '*' || tokens[i] == '/')

            {

                // While top of 'ops' has same or greater precedence to current

                // token, which is an operator. Apply operator on top of 'ops'

                // to top two elements in values stack

                while (!ops.empty() && hasPrecedence(tokens[i], ops.peek()))

                  values.push(applyOp(ops.pop(), values.pop(), values.pop()));

 

                // Push current token to 'ops'.

                ops.push(tokens[i]);

            }

        }

 

        // Entire expression has been parsed at this point, apply remaining

        // ops to remaining values

        while (!ops.empty())

            values.push(applyOp(ops.pop(), values.pop(), values.pop()));

 

        // Top of 'values' contains result, return it

        return values.pop();

    }

 

    // Returns true if 'op2' has higher or same precedence as 'op1',

    // otherwise returns false.

    public static boolean hasPrecedence(char op1, char op2)

    {

        if (op2 == '(' || op2 == ')')

            return false;

        if ((op1 == '*' || op1 == '/') && (op2 == '+' || op2 == '-'))

            return false;

        else

            return true;

    }

 

    // A utility method to apply an operator 'op' on operands 'a'

    // and 'b'. Return the result.

    public static int applyOp(char op, int b, int a)

    {

        switch (op)

        {

        case '+':

            return a + b;

        case '-':

            return a - b;

        case '*':

            return a * b;

        case '/':

            if (b == 0)

                throw new

                UnsupportedOperationException("Cannot divide by zero");

            return a / b;

        }

        return 0;

    }

 

    // Driver method to test above methods

    public static void main(String[] args)

    {

        System.out.println(EvaluateString.evaluate("10 + 2 * 6"));

        System.out.println(EvaluateString.evaluate("100 * 2 + 12"));

        System.out.println(EvaluateString.evaluate("100 * ( 2 + 12 )"));

        System.out.println(EvaluateString.evaluate("100 * ( 2 + 12 ) / 14"));

    }

}