/**
 *A function that takes a (nested) array as input and returns a flattened version of it
 *@param {Array} input - arbitrarily nested array of integers
 *@param {Array} input - progressively built flattened output array
 *@returns {Array} the flattened array, empty array if the input is either not an array or an empty array
 */
var flatten = function(input, output) {
    if (!input || !input.length) return [];
    output = output || [];
    for (var i = 0; i < input.length; i++) {
      var curr = input[i];
      if (Array.isArray (curr)) {
          flatten(curr, output);
      }
      else {
          output[output.length++] = curr;
      }
    }
    return output;
};