@return the most common filename which was used.

$*$

$*$ photographers $=[$

$*$ Photographer$($photos$=[$

$*$ Photo$($filename$=$"photo$1"),$

$*$ Photo$($filename$=$"photo$2"),$

$*$ Photo$($filename$=$"photo$3"),$

$*]),$

$*$ Photographer$($photos$=[$

$*$ Photo$($filename$=$"photo$4"),$

$*$ Photo$($filename$=$"photo$5"),$

$*$ Photo$($filename$=$"photo$6"),$

$*]),$

$*$ Photographer$($photos$=[$

$*$ Photo$($filename$=$"photo$2"),$

$*]),$

$*]$

$*$

$*$ mostCommonName$()$ should return Photo$($filename$=$"photo$2")$

$*$

$*/$

public Photo mostCommonName$()\{$

$//$ To be completed

return null;

$\}$