逆波兰(Reverse Polish Notation)

类似这样的后缀表达式: 叫做逆波兰表达式
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
["4", "13", "5", "/", "+"] -> (4 + (5 / 13)) -> 4

编译器比较喜欢从栈(stack)里面 pop 两个对象出来计算 然后 继续push 回去, 然后继续这样子计算下去..
算法也很简单

　　　　　public static void main(String[] args) {
		System.out.println(calRPN("4", "13", "5", "/", "+"));
	}
	
	public static int calRPN(String... tokens) {
		int ret = 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());
				int v = 0;
				
				if (t.equals("+")) {
					v = a + b;
				}else if (t.equals("-")) {
					v = a - b;
				}else if (t.equals("*")) {
					v = a * b;
				}else if (t.equals("/")) {
					v = a / b;
				}
				stack.push(String.valueOf(v));
			}
		}
		ret = Integer.valueOf(stack.pop());
		return ret;
	}