package com.hit.basmath.learn.queue_stack;

import java.util.Stack;

/**
 * 150. Evaluate Reverse Polish Notation
 * <p>
 * Evaluate the value of an arithmetic expression in Reverse Polish Notation.
 * <p>
 * Valid operators are +, -, *, /. Each operand may be an integer or another expression.
 * <p>
 * Note:
 * <p>
 * 1. Division between two integers should truncate toward zero.
 * 2. The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.
 * <p>
 * Example 1:
 * <p>
 * Input: ["2", "1", "+", "3", "*"]
 * Output: 9
 * Explanation: ((2 + 1) * 3) = 9
 * <p>
 * Example 2:
 * <p>
 * Input: ["4", "13", "5", "/", "+"]
 * Output: 6
 * Explanation: (4 + (13 / 5)) = 6
 * <p>
 * Example 3:
 * <p>
 * Input: ["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
 */
public class _150 {
    public int evalRPN(String[] tokens) {
        int a, b;
        Stack<Integer> stack = new Stack<Integer>();
        for (String token : tokens) {
            switch (token) {
                case "+":
                    stack.add(stack.pop() + stack.pop());
                    break;
                case "/":
                    b = stack.pop();
                    a = stack.pop();
                    stack.add(a / b);
                    break;
                case "*":
                    stack.add(stack.pop() * stack.pop());
                    break;
                case "-":
                    b = stack.pop();
                    a = stack.pop();
                    stack.add(a - b);
                    break;
                default:
                    stack.add(Integer.parseInt(token));
                    break;
            }
        }
        return stack.pop();
    }
}