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.
如果S2从1开始，那么就是26进制数和10进制数转换问题了，把S2加1则转化为这个问题，然后求解，求解方法见：
http://www.cnblogs.com/lavezhang/archive/2012/05/14/2499000.html