function solve() {

    return function(selector, defaultLeft, defaultRight) {

        var itemsLeft = [];
        var itemsRight = [];

        if(Array.isArray(defaultLeft) && defaultLeft.length > 0) {
            if(isArrayOfStrings(defaultLeft)) {
                itemsLeft = defaultLeft;
            }
        }

        if(Array.isArray(defaultRight) && defaultRight.length > 0) {
            if(isArrayOfStrings(defaultRight)) {
                itemsRight = defaultRight;
            }
        }

        var root = document.querySelector(selector);
        var div = document.createElement('div');
        var input = document.createElement('input');
        var button = document.createElement('button');
        var label = document.createElement('label');
        var ol = document.createElement('ol');
        var li = document.createElement('li');
        var img = document.createElement('img');

        var container = div.cloneNode();

        var leftCol = div.cloneNode();
        var leftSelect = div.cloneNode();
        var leftRadio = input.cloneNode();
        var leftLabel = label.cloneNode();

        var rightCol = div.cloneNode();
        var rightSelect = div.cloneNode();
        var rightRadio = input.cloneNode();
        var rightLabel = label.cloneNode();

        var inputBox = input.cloneNode();
        var inputBtn = button.cloneNode();

        container.className = 'column-container';

        leftCol.className = 'column';
        leftSelect.className = 'select';
        setAttributes(leftRadio, {'type' : 'radio', 'name' : 'column-select', 'id' : 'select-left-column', 'checked' : 'checked'});
        leftLabel.setAttribute('for', 'select-left-column');
        leftLabel.innerHTML = 'Add here';

        rightCol.className = leftCol.className;
        rightSelect.className = leftSelect.className;
        setAttributes(rightRadio, {'type' : 'radio', 'name' : 'column-select', 'id' : 'select-right-column'});
        rightLabel.setAttribute('for', 'select-right-column');
        rightLabel.innerHTML = leftLabel.innerHTML;

        setAttributes(inputBox, {'size' : '40', 'autofocus' : ''});
        inputBtn.innerHTML = 'Add';

        leftSelect.appendChild(leftRadio);
        leftSelect.appendChild(leftLabel);
        leftCol.appendChild(leftSelect);
        container.appendChild(leftCol);

        rightSelect.appendChild(rightRadio);
        rightSelect.appendChild(rightLabel);
        rightCol.appendChild(rightSelect);
        container.appendChild(rightCol);

        root.appendChild(container);
        root.appendChild(inputBox);
        root.appendChild(inputBtn);

        inputBtn.addEventListener('click', addItem);
        document.addEventListener('keydown', handleKeydown);

        buildLists();

        function handleKeydown(event) {
            var key = event.keyCode || event.which;
            if(key === 13) {
                addItem(event);
            }
        }

        function addItem(event) {
            if(isInputEmpty()) {
                return;
            }

            var item = inputBox.value;
            item = item.trim();

            if(item === '' || item.length < 1 || item === null) {
                return;
            }

            if(whichRadioIsChecked() === 0) {
                itemsLeft.push(item);
            } else {
                itemsRight.push(item);
            }

            buildLists();
            clearInput();
        }

        function deleteItem(event) {
            var targetLi = event.target.parentNode;
            var targetOl = event.target.parentNode.parentNode;

            if(targetOl.id === 'left_list') {
                inputBox.value = itemsLeft[parseInt(targetLi.id)];
                itemsLeft.splice(parseInt(targetLi.id),1);
            } else if(targetOl.id === 'right_list') {
                inputBox.value = itemsRight[parseInt(targetLi.id)];
                itemsRight.splice(parseInt(targetLi.id),1);
            }

            buildLists();
        }

        function buildLists() {
            var columns = document.getElementsByClassName('column');

            for (var i = 0; i < columns.length; i += 1) {
                clearList(i);
                buildList(i);
            }
        }

        function buildList(whichOne) {
            var data;
            var list = ol.cloneNode();

            if(whichOne === 0) {
                data = itemsLeft;
                list.setAttribute('id', 'left_list');
            } else if(whichOne === 1) {
                data = itemsRight;
                list.setAttribute('id', 'right_list');
            } else {
                return;
            }

            data.forEach((item, index) => {
                var entry = li.cloneNode();
                entry.innerHTML = item;
                entry.className = 'entry';

                if(whichOne === 0) {
                    entry.setAttribute('id', index + 'entryLeft');
                } else if (whichOne === 1) {
                    entry.setAttribute('id', index + 'entryRight');
                }

                var deleteBtn = img.cloneNode();
                deleteBtn.className = 'delete';
                deleteBtn.setAttribute('src', 'imgs/Remove-icon.png');
                deleteBtn.addEventListener('click', deleteItem);

                entry.appendChild(deleteBtn);
                list.appendChild(entry);
            });

            var columns = document.getElementsByClassName('column');
            columns[whichOne].appendChild(list);
        }

        function clearList(whichOne) {
            if(whichOne !== 1 && whichOne !== 0) {
                return;
            }

            var columns = document.getElementsByClassName('column');
            var column = columns[whichOne];

            for(var n = 0; n < column.childNodes.length; n += 1) {
                if (column.childNodes[n].tagName === 'OL') {
                    column.removeChild(column.childNodes[n]);
                }
            }
        }

        function isInputEmpty() {
            if(inputBox.value === '' || inputBox.value.length < 1 || inputBox.value === null) {
                return true;
            }
            return false;
        }

        function clearInput() {
            inputBox.value = '';
        }

        function isArrayOfStrings(array) {
            array.forEach(item => {
                if(typeof(item) !== 'string') {
                    return false;
                }
            });
            return true;
        }

        function whichRadioIsChecked() { // returns: 0 if left, 1 if right
            if(document.getElementById('select-left-column').checked) {
                return 0;
            } else if(document.getElementById('select-right-column').checked) {
                return 1;
            }
        }

        function setAttributes(DOMelement, attributes) {
            for(var key in attributes) {
                DOMelement.setAttribute(key, attributes[key]);
            }
        }

    };
}

// SUBMIT THE CODE ABOVE THIS LINE

if(typeof module !== 'undefined') {
    module.exports = solve;
}