Taking the frequency suffix sum of the number of 2 's present  after every index
for i in range ( n - 2 , - 1 , - 1 ) : NEW_LINE INDENT if ( a [ i ] == 2 ) : NEW_LINE INDENT suff [ i ] = suff [ i + 1 ] + 1 NEW_LINE DEDENT else : NEW_LINE INDENT suff [ i ] = suff [ i + 1 ] NEW_LINE DEDENT DEDENT