Question

Stack

Time Limit : 1 sec, Memory Limit : 131072 KB
English / Japanese  

Reverse Polish notation is a notation where every operator follows all of its operands. For example, an expression (1+2)*(5+4) in the conventional Polish notation can be represented as 1 2 + 5 4 + * in the Reverse Polish notation. One of advantages of the Reverse Polish notation is that it is parenthesis-free.

Write a program which reads an expression in the Reverse Polish notation and prints the computational result.

An expression in the Reverse Polish notation is calculated using a stack. To evaluate the expression, the program should read symbols in order. If the symbol is an operand, the corresponding value should be pushed into the stack. On the other hand, if the symbols is an operator, the program should pop two elements from the stack, perform the corresponding operations, then push the result in to the stack. The program should repeat this operations.

Input

An expression is given in a line. Two consequtive symbols (operand or operator) are separated by a space character.

You can assume that +, - and * are given as the operator and an operand is a positive integer less than 10⁶

Output

Print the computational result in a line.

Constraints

2 ≤ the number of operands in the expression ≤ 100
1 ≤ the number of operators in the expression ≤ 99
-1 × 10⁹ ≤ values in the stack ≤ 10⁹

Sample Input 1

1 2 +

Sample Output 1

3

Sample Input 2

1 2 + 3 4 - *

Sample Output 2

-3

USE JAVA

Answer 1

CODE IN JAVA:

Test.java file:

// Java proram to evaluate value of a postfix expression

import java.util.Stack;
import java.util.Scanner;
public class Test
{
   // It is a Method to evaluate value of a postfix expression
   static int postfixEvaluation(String expr)
   {
       //create a stack
       Stack<Integer> stack=new Stack<>();
      
       // Scan all characters one by one
       for(int i=0;i<expr.length();i++)
       {
           char ch=expr.charAt(i);
          
           // If the scanned character is an operand (number here),
           // push it to the stack.
           if(Character.isDigit(ch))
           stack.push(ch - '0');
           else if(ch==' ') {
              
           }
          
           // If the scanned character is an operator, pop two
           // elements from stack apply the operator
           else
           {
               int val1 = stack.pop();
               int val2 = stack.pop();
              
               switch(ch)
               {
                   case '+':
                   stack.push(val2+val1);
                   break;
                  
                   case '-':
                   stack.push(val2- val1);
                   break;
                  
                   case '/':
                   stack.push(val2/val1);
                   break;
                  
                   case '*':
                   stack.push(val2*val1);
                   break;
                   default:
                       break;
           }
           }
       }
       return stack.pop();     
   }
  
  
   public static void main(String[] args)
   {
       Scanner sc = new Scanner(System.in);
       System.out.print("Enter the postfix expression:");
       String expr=sc.nextLine();
       System.out.println("postfix expression is:"+expr);
       System.out.println("postfix evaluation: "+postfixEvaluation(expr));
   }
}

OUTPUT: