电话号码的字母组合

/**
 * Problem: Given a string containing digits from 2-9, return all possible letter combinations that the number could represent.
 * Approach: Use Java Stream and recursive function to generate all possible combinations.
 */
 
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
 
public class Solution {
    // Mapping of digits to corresponding letters
    private static final String[] KEYPAD = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
 
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.isEmpty()) {
            return List.of();
        }
        return letterCombinations(digits, 0).collect(Collectors.toList());
    }
 
    private Stream<String> letterCombinations(String digits, int index) {
        if (index == digits.length()) {
            return Stream.of("");
        }
        String letters = KEYPAD[digits.charAt(index) - '0'];
        return letters.chars()
                .mapToObj(c -> (char) c)
                .flatMap(letter -> letterCombinations(digits, index + 1).map(combination -> letter + combination));
    }
}