Editorial for CPC '21 Contest 1 P0 - AQT and Alphabet
Submitting an official solution before solving the problem yourself is a bannable offence.
One way to solve this problem is to loop through all the letters from
z, and check if any of them are not contained within the string ~S~. Once we find a letter, we will break from the loop and output it.
As we are looping in alphabetical order, this guarantees that the first letter we find will be the lowest alphabetically.