similar q was recently asked in Microsft code 4 bills contest held in india
the q there was to find the minimum length of string that makes all permutations of first 9 letters of english alphabet(taking 9 at a time) : a,b,c,d,e,f,g,h,i
eg abbaa is the minimum length of string that makes all possible pairs of alphabets(a,b) taking 2 at a time(aa,ab,ba,bb)
i thought a lot a lot abt this q but couldnt get the soln