leetcode-17-java

Given a string containing digits from 2-9 inclusive, 
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. Note that 1 does not map to any letters.
Example:

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

private static List<String> baseList = new ArrayList<String>(){{
    add(null);
    add(null);
    add("abc");
    add("def");
    add("ghi");
    add("jkl");
    add("mno");
    add("pqrs");
    add("tuv");
    add("wxyz");
}};

public static List<String> letterCombinations(String digits) {
    List<String> list = new ArrayList<>();
    if (null == digits || digits.length() == 0) {
        return list;
    }


    for (int i = 0; i < digits.length(); i++) {
        char[] chars = baseList.get(Integer.parseInt(String.valueOf(digits.charAt(i)))).toCharArray();
        if (i == 0) {
            for (char c : chars) {
                list.add(String.valueOf(c));
            }
        } else {
            int len = list.size();
            for (int j = 0; j < len; j++) {
                for (char c : chars) {
                    list.add(list.get(j) + c);
                }
            }
            list = list.subList(len, list.size());
        }

    }
    return list;
}