sstalin

/**
 * Function to flatten array of arrays of integers using recursion and Array.prototype.reduce.
 * @param {Array} an input array of arrays of integers
 * @returns {Array} flatten array of integers
 */
function flatten(arr){
    return arr.reduce(function (prev, curr) {
        return prev.concat(curr.constructor === Array ? flatten(curr) : curr);
    }, []);
};