String concatenation may take a lot of time, as it copies the whole string a, add a char, then write the resulting string to a (This actually depends on some factor, such as the language, compiler, etc, I dont really understand about that too). Instead of building the string ans, you can decide whether an answer exist (the d value in your code) and if it does, print the output as you go through the array cnt[] twice, one for the first half of the answer, and the other for the second half.

Relevant blog : https://mirror.codeforces.com/blog/entry/94701

Edit : Added a link to relevant blog