Implement four string-processing functions

You are given four independent string/list problems. Implement each as a separate function. ## 1) Filter strings by shared characters **Input:** - `words`: a list of strings - `allowed`: a string **Task:** Return a new list containing only those strings in `words` for which **at least one character** in the word appears somewhere in `allowed`. **Notes/assumptions:** - Treat characters literally (e.g., case-sensitive unless you normalize explicitly). - Order of returned strings should match the original order. **Example:** - `words = ["abc", "xyz", "bzz", "DEF"]`, `allowed = "qb"` → output `[