Thinking process - Backtracking recursive

O(m^n), n is the length of input string, m is the length of letters mapped with the digits. O(branches^depth)

public class Solution {
    private Map<Character, String> map = new HashMap<Character, String>(){ {
            put('1', ""); put('2', "abc"); put('3', "def"); put('4', "ghi"); put('5', "jkl"); put('6', "mno"); put('7', "pqrs");
            put('8', "tuv"); put('9', "wxyz");
        }
    };

    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<>();
        if (digits == null || digits.length() == 0) return result;
        dfsHelper(digits.toCharArray(), result, new StringBuilder(), 0);
        return result;
    }

    private void dfsHelper(char[] digits, List<String> result, StringBuilder sb, int index) {
        if (index >= digits.length) {
            result.add(sb.toString());
            return;
        }

        String letters = map.get(digits[index]);

        for (char c : letters.toCharArray()) {
            sb.append(c);
            dfsHelper(digits, result, sb, index + 1);
            sb.deleteCharAt(sb.length() - 1);
        }
    }
}

Thinking process - iterative FIFO

public class Solution {
    public List<String> letterCombinations(String digits) {
        LinkedList<String> result = new LinkedList<>();
        if (digits == null || digits.length() == 0) return result;

        String[] strs = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        result.add("");
        for (char c : digits.toCharArray()) {
            String letters = strs[c - '0'];
            int size = result.size();
            for (int i = 0; i < size; ++i) {
                String s = result.removeFirst();
                for (char letter : letters.toCharArray()) {
                    result.addLast(s + letter);
                }
            }
        }

        return result;
    }
}