Amazon "Bar Raiser" Interview Question

Given the sequence S1 = {a,b,c,d,…,x,y,z,aa,ab,ac…. } and given that this sequence corresponds (term for term) to the sequence S2 = {0,1,2,3,….}. Write code to convert an element of S2 to the corresponding element of S1.

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http://shashank7s.blogspot.com/2011/03/microsoft-excel-sheet-question-very.html