combinatorics/prufer: Added ranking and unranking of Prufer codes.
combinatorics/prufer: Added ranking and unranking of Prufer codes.

Signed-off-by: Saptarshi Mandal <13ec3c7bab52575f3a0cda6406c76269cc13b041@gmail.com>

maniteja123/sympy,lindsayad/sympy,ga7g08/sympy,Designist/sympy,shikil/sympy,postvakje/sympy,atsao72/sympy,hargup/sympy,jbbskinny/sympy,kumarkrishna/sympy,kmacinnis/sympy,asm666/sympy,Gadal/sympy,Designist/sympy,maniteja123/sympy,yashsharan/sympy,vipulroxx/sympy,shikil/sympy,Shaswat27/sympy,pandeyadarsh/sympy,kevalds51/sympy,vipulroxx/sympy,Curious72/sympy,jamesblunt/sympy,AkademieOlympia/sympy,beni55/sympy,AkademieOlympia/sympy,abloomston/sympy,drufat/sympy,kevalds51/sympy,shipci/sympy,oliverlee/sympy,MechCoder/sympy,aktech/sympy,ChristinaZografou/sympy,grevutiu-gabriel/sympy,moble/sympy,sampadsaha5/sympy,souravsingh/sympy,toolforger/sympy,postvakje/sympy,kmacinnis/sympy,emon10005/sympy,diofant/diofant,abloomston/sympy,kumarkrishna/sympy,Shaswat27/sympy,mcdaniel67/sympy,yukoba/sympy,mafiya69/sympy,hrashk/sympy,jamesblunt/sympy,saurabhjn76/sympy,farhaanbukhsh/sympy,emon10005/sympy,chaffra/sympy,Mitchkoens/sympy,aktech/sympy,bukzor/sympy,kaushik94/sympy,Davidjohnwilson/sympy,liangjiaxing/sympy,kaushik94/sympy,lindsayad/sympy,saurabhjn76/sympy,pandeyadarsh/sympy,yukoba/sympy,postvakje/sympy,atreyv/sympy,moble/sympy,ahhda/sympy,wyom/sympy,cccfran/sympy,garvitr/sympy,drufat/sympy,pandeyadarsh/sympy,drufat/sympy,wanglongqi/sympy,kevalds51/sympy,hargup/sympy,kumarkrishna/sympy,Sumith1896/sympy,shipci/sympy,jerli/sympy,lidavidm/sympy,Arafatk/sympy,sahilshekhawat/sympy,beni55/sympy,maniteja123/sympy,Curious72/sympy,cccfran/sympy,ga7g08/sympy,sahmed95/sympy,AunShiLord/sympy,yukoba/sympy,asm666/sympy,wanglongqi/sympy,dqnykamp/sympy,sahilshekhawat/sympy,flacjacket/sympy,ChristinaZografou/sympy,debugger22/sympy,Sumith1896/sympy,meghana1995/sympy,Davidjohnwilson/sympy,chaffra/sympy,skidzo/sympy,Vishluck/sympy,VaibhavAgarwalVA/sympy,VaibhavAgarwalVA/sympy,pbrady/sympy,mcdaniel67/sympy,cccfran/sympy,toolforger/sympy,kaichogami/sympy,lidavidm/sympy,emon10005/sympy,ahhda/sympy,Mitchkoens/sympy,AkademieOlympia/sympy,hargup/sympy,Gadal/sympy,cswiercz/sympy,atreyv/sympy,sahilshekhawat/sympy,mafiya69/sympy,mcdaniel67/sympy,shikil/sympy,hrashk/sympy,MridulS/sympy,bukzor/sympy,liangjiaxing/sympy,atreyv/sympy,hrashk/sympy,lidavidm/sympy,jaimahajan1997/sympy,MechCoder/sympy,jerli/sympy,Vishluck/sympy,sahmed95/sympy,jerli/sympy,Arafatk/sympy,oliverlee/sympy,Titan-C/sympy,rahuldan/sympy,liangjiaxing/sympy,iamutkarshtiwari/sympy,iamutkarshtiwari/sympy,debugger22/sympy,meghana1995/sympy,aktech/sympy,kaichogami/sympy,garvitr/sympy,rahuldan/sympy,Designist/sympy,oliverlee/sympy,wanglongqi/sympy,beni55/sympy,meghana1995/sympy,abhiii5459/sympy,sampadsaha5/sympy,grevutiu-gabriel/sympy,asm666/sympy,vipulroxx/sympy,farhaanbukhsh/sympy,wyom/sympy,cswiercz/sympy,MechCoder/sympy,garvitr/sympy,rahuldan/sympy,Mitchkoens/sympy,dqnykamp/sympy,sampadsaha5/sympy,ahhda/sympy,MridulS/sympy,AunShiLord/sympy,sunny94/temp,pbrady/sympy,kmacinnis/sympy,yashsharan/sympy,iamutkarshtiwari/sympy,madan96/sympy,amitjamadagni/sympy,skidzo/sympy,jaimahajan1997/sympy,toolforger/sympy,sunny94/temp,cswiercz/sympy,wyom/sympy,skidzo/sympy,VaibhavAgarwalVA/sympy,Titan-C/sympy,chaffra/sympy,srjoglekar246/sympy,yashsharan/sympy,souravsingh/sympy,saurabhjn76/sympy,shipci/sympy,jbbskinny/sympy,moble/sympy,farhaanbukhsh/sympy,madan96/sympy,abhiii5459/sympy,pbrady/sympy,jamesblunt/sympy,atsao72/sympy,abhiii5459/sympy,Vishluck/sympy,mafiya69/sympy,abloomston/sympy,sahmed95/sympy,sunny94/temp,AunShiLord/sympy,Shaswat27/sympy,dqnykamp/sympy,Davidjohnwilson/sympy,ChristinaZografou/sympy,MridulS/sympy,Titan-C/sympy,lindsayad/sympy,grevutiu-gabriel/sympy,Curious72/sympy,ga7g08/sympy,jaimahajan1997/sympy,souravsingh/sympy,Gadal/sympy,jbbskinny/sympy,amitjamadagni/sympy,kaushik94/sympy,skirpichev/omg,bukzor/sympy,atsao72/sympy,madan96/sympy,kaichogami/sympy,Sumith1896/sympy,Arafatk/sympy,debugger22/sympy