Finish the solution so that it takes an input `n` (integer) and returns a string that is the decimal representation of the number grouped by commas after every 3 digits.
Assume: `0 <= n < 2147483647`
## Examples
```
       1  ->           "1"
      10  ->          "10"
     100  ->         "100"
    1000  ->       "1,000"
   10000  ->      "10,000"
  100000  ->     "100,000"
 1000000  ->   "1,000,000"
35235235  ->  "35,235,235"
```
def group_by_commas(n):
    return '{:,}'.format(n)
def group_by_commas(n):
    return '{:,}'.format(n)