Evaluate Reverse Polish Notation

 You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.


Evaluate the expression. Return an integer that represents the value of the expression.


Note that:


The valid operators are '+', '-', '*', and '/'.

Each operand may be an integer or another expression.

The division between two integers always truncates toward zero.

There will not be any division by zero.

The input represents a valid arithmetic expression in a reverse polish notation.

The answer and all the intermediate calculations can be represented in a 32-bit integer.

 


Example 1:


Input: tokens = ["2","1","+","3","*"]

Output: 9

Explanation: ((2 + 1) * 3) = 9

Example 2:


Input: tokens = ["4","13","5","/","+"]

Output: 6

Explanation: (4 + (13 / 5)) = 6

Example 3:


Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]

Output: 22

Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5

= ((10 * (6 / (12 * -11))) + 17) + 5

= ((10 * (6 / -132)) + 17) + 5

= ((10 * 0) + 17) + 5

= (0 + 17) + 5

= 17 + 5

= 22

 


Constraints:


1 <= tokens.length <= 104

tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].



class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> st = new Stack<>();

        for (String token : tokens) {
            // If token is an operator
            if (token.equals("+") || token.equals("-") ||
                token.equals("*") || token.equals("/")) {

                int b = st.pop(); // 2nd operand
                int a = st.pop(); // 1st operand

                switch (token) {
                    case "+": st.push(a + b); break;
                    case "-": st.push(a - b); break;
                    case "*": st.push(a * b); break;
                    case "/": st.push(a / b); break; // Integer division
                }
            } else {
                // Token is a number
                st.push(Integer.parseInt(token));
            }
        }

        return st.pop(); // Final result
    }
}

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