code: function difference(arrayA, arrayB) {
    var maxLength = _.max([ arrayA.length, arrayB.length ]);
  
    for (var i = 0, j = 0; i < maxLength; i += 1, j += 1) {
      if (arrayA[i] !== arrayB[j]) {
        if (arrayB[i-1] === arrayA[i]) {
          j -= 1;
        } else if (arrayB[i+1] === arrayA[i]) {
          j += 1;
        } else {
          return arrayA[i];
        }
      }
    }
  }
docs: Given two arrays of backbone models, with at most one model added and one model removed from each, return the model in arrayA that is not in arrayB or undefined.