No. Please re-read the problem statement. It's way more complicated than that. If you figure out how to do it, try to figure out how to do it in O(N).
An optimal solution would be something like
- create an array of 26 or 52 bytes depending on whether this is case sensitive
- iterate over the string and set the byte corresponding to each letter’s position in the alphabet to 1
- iterate over the byte array and for each 1 you encounter print out the corresponding letter
But that's what "lexicographical" means?
> In mathematics, the lexicographical order is a generalization of the way words are alphabetically ordered based on the alphabetical order of their component letters.
> This generalization consists primarily in defining a total order on the sequences (often called strings in computer science) of elements of a finite totally ordered set, often called an alphabet.
Consider the input “ba”
This solution will return “ab”, which is not a strong that can be generated by deleting characters from the input.