public boolean evaluate(final String expr) {
		Stack stack = new Stack();
		if(expr == null) {
			return false;
		}
		
		char[] chars = expr.toCharArray();
		for(char character : chars) {
			if(character == '(') {
				StackNode node = new StackNode();
				node.setData(character);
				stack.push(node);
			} else if(character == ')') {
				if(stack.isEmpty()) {
					return false;
				} 
				stack.pop();
			}
		}
		
		if(stack.isEmpty()) {
			return true;
		}
		return false;
	}