public static String infixToPostFix(String input) throws IllegalArgumentException {
		Stack<String> stack = new Stack<String>();
		String[] arr = input.split(" ");
		String output = "";

		for (int i = 0; i < arr.length; i++) {
			if (!isOperator(arr[i])) {
				output += arr[i] + " ";
				continue;
			}

			output = pushOperator(stack, arr[i], output);

		}
		while (!stack.isEmpty()) {
			output += stack.pop() + " ";
		}

		return output;
	}

	public static String pushOperator(Stack<String> stack, String operator, String output) {
		String out = output;

		if (stack.size() == 0) {
			stack.push(operator);
			return output;
		}

		if (operator.equals(")")) {
			while (!stack.peek().equals("(")) {
				out += stack.pop() + " ";
				if (stack.isEmpty())
					return out;
			}
			stack.pop();
			return out;
		}

		while (precedence(stack.peek()) >= precedence(operator) && !stack.peek().equals("(")) {
			out += stack.pop() + " ";
			if (stack.isEmpty()) {
				break;
			}
		}
		stack.push(operator);
		return out;
	}

	public static int precedence(String operator) {
		switch (operator) {
		case ("+"):
			return 1;
		case ("-"):
			return 1;
		case ("*"):
			return 2;
		case ("/"):
			return 2;
		case ("^"):
			return 3;
		case ("("):
			return 4;
		}
		return -1;
	}

	public static boolean isOperator(String input) {
		return input.matches("[\\+\\-\\*\\/\\(\\)]");
	}