1  /*
 2   * We will create a stack and push elements into it (if they are numbers)
 3   * If they are operands, we will pop the previou 2 values and apply the operation and push the output
 4   */ 
 5  
 6  import java.util.Stack;
 7  import java.util.Scanner;
 8  
 9  public class postfix_expression {
10    public static int evalPostfix(String[] arr) {
11      Stack<Integer> stack = new Stack<>();
12  
13      for(String token : arr) {
14        if(token.matches("-?\\d+")) {         // This regex expression checks if the token is a digit
15          stack.push(Integer.parseInt(token));
16        }
17  
18        else {
19          int val1 = stack.pop();
20          int val2 = stack.pop();
21  
22          switch (token) {
23            case "+":
24              stack.push(val1 + val2);
25              break;
26  
27            case "-" :
28              stack.push(val2 - val1);
29              break;
30  
31            case "*" :
32              stack.push(val1 * val2);
33              break;
34  
35            case "/" :
36              stack.push(val2 / val1);
37          
38            default:
39              break;
40          }
41        }
42      }
43      return stack.pop();
44    }
45    public static void main(String[] args) {
46      System.out.println("Enter the expression: ");
47      Scanner sc = new Scanner(System.in);
48      
49      String expression = sc.nextLine();
50      String[] array_elements = expression.split("");
51  
52      System.out.println(evalPostfix(array_elements));
53    }
54  }