Find substring from dictionary concatenation

Given a string s and a dictionary of words dict, determine whether s contains a substring that is exactly a concatenation of one or more words from dict (words may be reused; order is arbitrary). Return the start index of any one such substring, or -1 if none exists. Words can have varying lengths. Discuss an efficient algorithm, its time and space complexity, and edge cases (e.g., overlapping matches, very large dict, unicode). Implement a function solve(s: string, dict: List[str]) -> int.