#150逆波兰表达式求值

import java.util.ArrayDeque;
import java.util.Deque;

final class Solution {
    public int evalRPN(String[] tokens) {
        Deque<Integer> operands = new ArrayDeque<>();
        for (String token : tokens) {
            if (!isOperator(token)) {
                operands.push(Integer.parseInt(token));
                continue;
            }
            int right = operands.pop();
            int left = operands.pop();
            int value = switch (token) {
                case "+" -> left + right;
                case "-" -> left - right;
                case "*" -> left * right;
                case "/" -> left / right;
                default -> throw new IllegalStateException("Unexpected operator");
            };
            operands.push(value);
        }
        return operands.pop();
    }

    private boolean isOperator(String token) {
        return token.length() == 1 && "+-*/".indexOf(token.charAt(0)) >= 0;
    }
}