Recursion Backtracking Combinatorics
Intermediate Level

Implement the letterCombinationsPhoneCount method that counts letter combinations generated from phone digits.

The input is a string digits containing phone keypad digits.

Each digit maps to a group of letters, like 2 -> abc, 3 -> def, and so on. Your task is to count how many letter combinations can be formed by choosing one letter for each digit.

For example, digits 23 can form 3 × 3 = 9 combinations. If the input is empty, return 0.

Example 1
Input:
digits (string) = 23
Return:
(int) 9
Example 2
Input:
digits (string) = 2
Return:
(int) 3
Example 3
Input:
digits (string) =
Return:
(int) 0

Use the size of each digit's letter group.

Every digit contributes a number of choices. For digits 2, 3, 4, 5, 6, and 8, there are 3 choices. For digits 7 and 9, there are 4 choices.

Multiply the choices for all digits to get the total number of combinations.

Pseudocode:

function letterCombinationsPhoneCount(digits):
    if digits is empty:
        return 0
    map = {
        '2': 3, '3': 3, '4': 3, '5': 3,
        '6': 3, '7': 4, '8': 3, '9': 4
    }
    count = 1
    for each digit in digits:
        count = count * map[digit]
    return count
Run your code to see the result.