Letter Combinations of a Phone Number

Given a string digits containing digits from 2 to 9, return all possible letter combinations that the number could represent using the standard telephone mapping. The mapping for the digits is given by: $$2\to\text{abc},\quad 3\to\text{def},\quad 4\to\text{ghi},\quad 5\to\text{jkl},\quad 6\to\text{mno},\quad 7\to\text{pqrs},\quad 8\to\text{tuv},\quad 9\to\text{wxyz}.$$

Your task is to write a program that reads the digit string from standard input (stdin) and prints all valid letter combinations separated by a single space on one line to standard output (stdout). If the input is an empty string, the program should output nothing.

inputFormat

The input is given on a single line from standard input containing a string of digits (each digit is between '2' and '9').

outputFormat

Output all possible letter combinations derived from the digit-to-letter mapping, separated by a single space on one line to standard output.

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