/**
  extensions is an Array and each item has such format:
  {firstName: 'xxx', lastName: 'xxx', ext: 'xxx', extType: 'xxx'}
  lastName, ext can be empty, extType can only has "DigitalUser", "VitrualUser","FaxUser","Dept","AO".
**/

/**
  Question 1: sort extensions by "firstName" + "lastName" + "ext" ASC
**/

/**
  Question 2: sort extensions by extType follow these orders ASC
  DigitalUser < VitrualUser < FaxUser < AO < Dept.
**/

/**
  saleItems is an Array has each item has such format:
  {
	month: n, //[1-12],
	date: n, //[1-31],
	transationId: "xxx",
	salePrice: number
  }
**/

/**
  Question 3: write a function to calculate and return a list of total sales (sum) for each quarter, expected result like:
  [
  	{quarter: 1, totalPrices: xxx, transactionNums: n},
  	{....}
  ]
**/

/**
  Question 4: write a function to calculate and return a list of average sales for each quarter, expected result like:
  [
    {quarter: 1, averagePrices: xxx, transactionNums: n},
    {....}
  ]
**/

/**
  Question 5: please create a tool to generate Sequence
  Expected to be used like:
  var sequence1 = new Sequence();
  sequence1.next() --> return 1;
  sequence1.next() --> return 2;
  
  in another module:
  var sequence2 = new Sequence();
  sequence2.next() --> 3;
  sequence2.next() --> 4;
**/

/**
    Question 6:
    AllKeys: 0-9;
    usedKeys: an array to store all used keys like [2,3,4];
    We want to get an array which contains all the unused keys,in this example it would be: [0,1,5,6,7,8,9]
**/