好玩的线上检测代码工具-codewars(1)

检测环境：
http://www.codewars.com
基本描述：

Task:

Your task is to write a function which returns the sum of following series upto nth term(parameter).

Series: 1 + 1/4 + 1/7 + 1/10 + 1/13 + 1/16 +…

Rules:

You need to round the answer to 2 decimal places and return it as String.
If the given value is 0 then it should return 0.00
You will only be given Natural Numbers as arguments.

Examples:

SeriesSum(1) => 1 = “1.00”
SeriesSum(2) => 1 + 1/4 = “1.25”
SeriesSum(5) => 1 + 1/4 + 1/7 + 1/10 + 1/13 = “1.57”
NOTE: In PHP the function is called series_sum().

function series_sum($n) {  // Your code here  if($n == 0)  {    return '0.00';  }  $res = 0;  $n = floor($n);  if($n == 1)  {      return '1.00';  }  else  {    for($i = 1;$i <= 3*$n-2; $i = $i+3)    {       $res += 1/$i;    }    return (String) round($res, 2);  }}