Given a string of digits 2–9, return all letter combinations the number could represent on a classic phone keypad (2=abc, 3=def, 4=ghi, 5=jkl, 6=mno, 7=pqrs, 8=tuv, 9=wxyz).
Input: one line, the digit string (may be empty). Output: one combination per line (nothing for an empty input). Any order — printed canonically.
23
ad ae af bd be bf cd ce cf
2
a b c
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