Leetcode 3435. Frequencies of Shortest Supersequences

Given a set of 2-letter words (bigrams) over ≤16 distinct letters, each bigram imposes a precedence constraint (a must appear before b) and the task is to find all shortest strings that satisfy every constraint (cycles may force repeated letters) and return the unique letter-frequency vectors for those shortest common supersequences, ignoring sequences that are permutations of each other.