Nirbhaya Case: How was the case cracked and put to trial so soon

On December 16, 2012, a woman (Nirbhaya) and her friend were returning to their homes after watching a film in Saket. The duo took Auto till Munirka and waited for a passenger bus till Palam. But little did they know that the bus did not have passengers but drunk men.
Soon after dirty comments about the girl, these six men raped the girl after beating her up brutally. 
But this case was cracked quickly by amazing investigation and the trial was done in a disciplined and consistent manner too.