public int evalRPN(String[] tokens) {
		//assume the length is greater than 3
		final int len = tokens.length;
		Integer result = new Integer(0);
		Stack stack = new Stack();
		for(int i = 0; i < len; i++){
			if(!isOperator(tokens[i]))
				stack.push(Integer.valueOf(tokens[i]));
			else{
				Integer operand2 = stack.pop();
				Integer operand1 = stack.pop();
				switch(tokens[i]){
				case "+" : result = operand1 + operand2; break;
				case "-" : result = operand1 - operand2; break;
				case "*" : result = operand1 * operand2; break;
				case "/" : result = operand1 / operand2; break;
				default : break;
				}
				stack.push(result);
			}
		}
		return stack.pop();
    }