https://prit.substack.com/p/hackerrank
Seems like a strange problem - it's not so much looking for substrings as checking if they contain any common characters. So, why not just (if (empty? (clojure.set/intersection (set s1) (set s2))) "NO" "YES") ?
(if (empty? (clojure.set/intersection (set s1) (set s2))) "NO" "YES")