给你一个字符串数组 tokens ，表示一个根据 逆波兰表示法 表示的算术表达式。

请你计算该表达式。返回一个表示表达式值的整数。

示例 1：

输入：tokens = ["2","1","+","3","*"]
输出：9
解释：该算式转化为常见的中缀算术表达式为：((2 + 1) * 3) = 9

示例 2：

输入：tokens = ["4","13","5","/","+"]
输出：6
解释：该算式转化为常见的中缀算术表达式为：(4 + (13 / 5)) = 6

import java.util.Stack;public class title150 {public static void main(String[] args) {String[] tokens = {"2", "1", "+", "3", "*"};System.out.println(evalRPN(tokens));}public static int evalRPN(String[] tokens) {Stack<Integer> stack = new Stack<>();Integer a, b, c;for (int i = 0; i < tokens.length; i++) {//1.先判断是否为四种符号String s = tokens[i];if (s.equals("+")) {//需要将字符串形式的数字转化为int型a = stack.pop();b = stack.pop();c = a + b;stack.push(c);} else if (s.equals("-")) {a = stack.pop();b = stack.pop();c = b - a;stack.push(c);} else if (s.equals("*")) {a = stack.pop();b = stack.pop();c = a * b;stack.push(c);} else if (s.equals("/")) {a = stack.pop();b = stack.pop();c = b / a;stack.push(c);} else {//先将字符串数字转换为int,再入栈stack.push(Integer.valueOf(s));}}return stack.pop();}
}