LeetCode Evaluate Reverse Polish Notation(用栈来计算后缀表达式)

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

public class Solution {
public int evalRPN(String[] tokens) {
String operators = "+-*/";
int result = 0;

Stack<String> stack = new Stack<String>();

for (String s: tokens) {
if (!operators.contains(s)) {
stack.push(s);
} else {
int b = Integer.valueOf(stack.pop());
int a = Integer.valueOf(stack.pop());
switch (s) {
case "+":
stack.push(String.valueOf(a + b));
break;
case "-":
stack.push(String.valueOf(a - b));
break;
case "*":
stack.push(String.valueOf(a * b));
break;
case "/":
stack.push(String.valueOf(a / b));
break;
}
}
}

result = Integer.valueOf(stack.pop());

return result;
}
}