LeetCode 17 - Letter Combinations of a Phone Number

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.
Solution 1:

private String[] btns={"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
public List<String> letterCombinations(String digits) {
    List<String> result = new ArrayList<>();
    if(digits == null || digits.isEmpty()) {
        result.add("");
        return result;
    }
    char[] letters = new char[digits.length()];
    combine(result, letters, digits.toCharArray(), 0);
    return result;
}

public void combine(List<String> result, char[] letters, char[] digits, int start) {
    if(start == letters.length) {
        result.add(new String(letters));
        return;
    }
    char[] chars = btns[digits[start]-'0'].toCharArray();
    for(char c:chars) {
        letters[start] = c;
        combine(result, letters, digits, start+1);
    }
}

 
Solution 2:

public List<String> letterCombinations(String digits) {
    List<String> result = new ArrayList<>();
    result.add("");
    if(digits == null || digits.isEmpty()) {
        return result;
    }
    String[] btns = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    for(int i=0; i<digits.length(); i++) {
        List<String> newResult = new ArrayList<>();
        char[] chars = btns[digits.charAt(i)-'0'].toCharArray();
        for(String s: result) {
            for(char c: chars) {
                newResult.add(s+c);
            }   
        }
        result = newResult;
    }
    return result;
}