how find the number of diffrent substrings in string S.
for clarity: if S=aa than it’s got 3 different substrings - empty, ‘a’ and ‘aa’?
If this question is related to the problem included in Sep 13 challenge, then please hold on till the contest gets over. You will be answered in a very better manner. Don’t worry. 
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if the string length is n… then total substrings are 2^n .
And how come, the tile of this question is “permutation and combination”?
he has asked for different substrings
updated 
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build the suffix array and compute the lcp(longest common prefix).