[leetcode]evaluate-reverse-polish-notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.
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

答案：

import java.util.*;
public class Solution {
    public int evalRPN(String[] tokens) {
        Stack stack = new Stack();  
          
        for(String s : tokens) {  
            if(s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/")) {  
                int second = Integer.parseInt(stack.pop());  
                int first = Integer.parseInt(stack.pop());  
                if(s.equals("+")) {  
                    stack.push(String.valueOf(first+second));  
                } else if(s.equals("-")) {  
                    stack.push(String.valueOf(first-second));  
                } else if(s.equals("*")) {  
                    stack.push(String.valueOf(first*second));  
                } else if(s.equals("/")) {  
                    stack.push(String.valueOf(first/second));  
                }  
            } else {  
                stack.push(s);  
            }  
        }  
          
        return Integer.parseInt(stack.pop());  
    }
}