Letter Combinations (Cartesian Product)

Given a string digits containing digits from 2–9 inclusive, return all possible letter combinations that the number could represent on a traditional telephone keypad mapping. Each digit maps to a set of letters as follows: 2→abc, 3→def, 4→ghi, 5→jkl, 6→mno, 7→pqrs, 8→tuv, 9→wxyz. You may return the combinations in any order. If digits is empty, return an empty list.