Java の後置式

Expression: 82*9+

Character | Stack       |  Explanation
-------------------------------------------
8           8             8 is an Operand and pushed to Stack
2           8 2           2 is an Operand and pushed to Stack 
*           16 (8*2)      * is an Operator. 
                          Poped 8 and 2 and multiply 
                          them. Lastly pushed the result to Stack 
9           16 9          9 is an Operand and pushed to Stack
+           25 (16+9)     + is an operator, 
                          Stack popped 12 and 9 and added them. 
                          Lastly pushed the result to Stack. 

Result: 25

import java.util.Stack;

public class Postfix {
  // Method to evaluate the postfix expression
  static int EvaluatePostfix(String EXP) {
    // Creating a stack
    Stack<Integer> EqStack = new Stack<>();

    // Scanning all the characters from the expression string
    for (int i = 0; i < EXP.length(); i++) {
      char ch = EXP.charAt(i);

      // If the character is an operator then push it to the stack
      if (Character.isDigit(ch))
        EqStack.push(ch - '0');

      // If the character is the operator, pop two elements for
      // the stack and performs the mathematical operation.
      else {
        int Val_1 = EqStack.pop(); // Getting the first element
        int Val_2 = EqStack.pop(); // Getting the second element

        switch (ch) {
          case '+':
            EqStack.push(Val_2 + Val_1); // Perform add operation
            break;

          case '-':
            EqStack.push(Val_2 - Val_1); // Perform subtraction operation
            break;

          case '/':
            EqStack.push(Val_2 / Val_1); // Perform divide operation
            break;

          case '*':
            EqStack.push(Val_2 * Val_1); // Perform multiply operation
            break;
        }
      }
    }
    return EqStack.pop();
  }

  public static void main(String[] args) {
    String EXP = "82*9+"; // The expression string
    System.out.println("Postfix evaluation result: " + EvaluatePostfix(EXP));
  }
}

Postfix evaluation result: 25