References: 
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-042j-mathematics-for-computer-science-spring-2005/lecture-notes/l9_graphs2.pdf
https://www.maa.org/sites/default/files/pdf/upload_library/22/Allendoerfer/1981/0025570x.di021114.02p00982.pdf
https://en.wikipedia.org/wiki/Tournament_(graph_theory)