How to check for a string that for every character in it, there exists all the characters which are alphabetically smaller than it before it  e.g aab  is correct while aacb is not, because the second case, we have 'c' but 'b' is not present before it. 
Also aac is not correct as it does not have 'b' before 'c'.
The idea is very simple, for each char in the string, it should not less than its preceding, and it shouldn't larger than its preceding + 1.
a string that for every character in it, there exists all the characters which are alphabetically smaller than it before it