逆波兰式

算法很简单
(a+b)*c-(a+b)/e的后缀表达式为：
(a+b)*c-(a+b)/e
→((a+b)*c)((a+b)/e)-
→((a+b)c*)((a+b)e/)-
→(ab+c*)(ab+e/)-
→ab+c*ab+e/-

public static int evalRPN(String[] tokens) {
		int returnValue = 0;
		String operators = "+-*/";
 
		Stack<String> stack = new Stack<String>();
 
		for (String t : tokens) {
			if (!operators.contains(t)) {
				stack.push(t);
			} else {
				int a = Integer.valueOf(stack.pop());
				int b = Integer.valueOf(stack.pop());
				switch (t) {
				case "+":
					stack.push(String.valueOf(a + b));
					break;
				case "-":
					stack.push(String.valueOf(b - a));
					break;
				case "*":
					stack.push(String.valueOf(a * b));
					break;
				case "/":
					stack.push(String.valueOf(b / a));
					break;
				}
			}
		}
 
		returnValue = Integer.valueOf(stack.pop());
 
		return returnValue;
	}