Letter Combinations of a Phone Number | Leetcode

Given a digit string, return all possible letter combinations that the number could represent.
A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

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Solution: For each letter, take its list of corresponding characters, and append to each sub-list of strings in a double for-loop. Watch out for boundary conditions where the subList is empty or where the list of corresponding characters is empty.

public class Solution {
    private static final Map<Character, List<Character>> digitToLettersMap = new HashMap<>();
    
    static {
        digitToLettersMap.put('0', Collections.emptyList());
        digitToLettersMap.put('1', Collections.emptyList());
        digitToLettersMap.put('2', Arrays.asList('a', 'b', 'c'));
        digitToLettersMap.put('3', Arrays.asList('d', 'e', 'f'));
        digitToLettersMap.put('4', Arrays.asList('g', 'h', 'i'));
        digitToLettersMap.put('5', Arrays.asList('j', 'k', 'l'));
        digitToLettersMap.put('6', Arrays.asList('m', 'n', 'o'));
        digitToLettersMap.put('7', Arrays.asList('p', 'q', 'r', 's'));
        digitToLettersMap.put('8', Arrays.asList('t', 'u', 'v'));
        digitToLettersMap.put('9', Arrays.asList('w', 'x', 'y', 'z'));
    }
    
    public List<String> letterCombinations(String digits) {
        if (digits.isEmpty()) return Collections.emptyList();
        
        // Recurse on next substring.
        List<String> subList = letterCombinations(digits.substring(1));
        
        // Get the list of characters corresponding to the current digit.
        List<Character> chars = digitToLettersMap.get(digits.charAt(0));
        
        // If current list is empty, skip and return subList.
        if (chars.isEmpty()) return subList;
        
        List<String> answer = new ArrayList<>();
        
        if (subList.isEmpty()) {
            // If the subList is empty, just ignore it and return current
            // list of characters as a list of strings.
            for (char c: chars) {
                answer.add(Character.toString(c));
            }
        } else {
            StringBuilder sb = new StringBuilder();
        
            for (char c: chars) {
                sb.append(c);
                for (String s: subList) {
                    answer.add(sb.append(s).toString());
                    sb.setLength(1);
                }
                sb.setLength(0);
            }
        }
        
        return answer;
    }
}