//Created by Amogh Kulkarni

private void fillTree() {
        //stack: > r > ^ q ! p p
        Deque<VariableNode> variableStack = new ArrayDeque<>();
        OperatorNode lastNode;

        tree = new AbstractSyntaxTree(getOperatorNodeFromString(rpnStack.pop()));

        while (!rpnStack.isEmpty()) {
            String token = rpnStack.peek();

            if (isOperator(token)) {
                lastNode = getOperatorNodeFromString(rpnStack.pop());

                if (!variableStack.isEmpty())
                    lastNode.setLeftOperand(variableStack.pop());
                else if (!isOperator(rpnStack.peek()))
                    lastNode.setLeftOperand(new VariableNode(rpnStack.peek()));
                else if (!rpnStack.isEmpty())
                    lastNode.setLeftOperand(getOperatorNodeFromString(rpnStack.peek()));

                if ("!".equals(token)) {
                    lastNode.setRightOperand(null);
                }
                else {
                    if (!variableStack.isEmpty())
                        lastNode.setRightOperand(variableStack.pop());
                    else if (!isOperator(rpnStack.peek()))
                        lastNode.setRightOperand(new VariableNode(rpnStack.peek()));
                    else if (!rpnStack.isEmpty())
                        lastNode.setRightOperand(getOperatorNodeFromString(rpnStack.peek()));
                }

                addToTree(tree, lastNode);
                continue;
            }

            if (Character.isAlphabetic(token.charAt(0))) {
                variableStack.push(new VariableNode(rpnStack.pop()));
            }
        }
    }