Evaluate the value of an arithmetic expression in .

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6 用stack, 每次遇到operator就从stack里取出2个数，后取出的数 运算符 先取出的数，然后再放回stack里。遇到数字直接放入stack里。switch不支持string，换成index。

public class Solution {    public int evalRPN(String[] tokens) {        Stack
     
       stack=new Stack
      
       ();        String operators="+-*/";        for(String t:tokens){            if(!operators.contains(t)){                stack.push(t);            }else{                int index=operators.indexOf(t);                int b=Integer.valueOf(stack.pop());                int a=Integer.valueOf(stack.pop());                                    switch(index){                case 0:                             stack.push(String.valueOf(a+b));                    break;                case 1:                    stack.push(String.valueOf(a-b));                    break;                case 2:                    stack.push(String.valueOf(a*b));                    break;                case 3:                    stack.push(String.valueOf(a/b));                    break;                          }            }        }                return Integer.valueOf(stack.pop());    }}