On a phone keypad, digits 2-9 each map to a set of letters: 2=abc, 3=def, 4=ghi, 5=jkl, 6=mno, 7=pqrs, 8=tuv, 9=wxyz. Given a string of such digits, generate every letter combination it could represent.

This entry exposes the recursive helper directly. Call dfs(startIndex, path, res, digits) to extend path by choosing one letter for digit digits[startIndex], appending complete combinations to res when startIndex reaches digits.length.

startIndex: current digit position

path: characters chosen so far

res: accumulator for completed combinations

digits: the digit string (or array)

Mutates res to contain every combination; the function itself returns nothing meaningful.

Digits use only 2-9.

At each level, branch over every letter mapped to the current digit.