Letter Combinations of a Phone Number

Java

public class Solution {
    /**
     * @param digits: A digital string
     * @return: all posible letter combinations
     */
    public List<String> letterCombinations(String digits) {
        // write your code here
        /*for (int i = 'a'; i <= 'c'; i++) {
            for (int j = 'j'; j <= 'l'; j++) {
                for (int k = 'd'; k <= 'f'; k++) {
                    print();
                }
            }
        }*/
        
        String[] phone = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        
        List<String> ans = new ArrayList<>();
        if (digits == null || digits.length() == 0) {
            return ans;
        }
        
        dfs(0, "", digits, phone, ans);
        return ans;
    }
    
    private void dfs(int x, String str, String digits, String[] phone, List<String> ans) {
        if (x == digits.length()) {
            ans.add(str);
            return;
        }
        
        int d = digits.charAt(x) - '0';
        for (char c : phone[d].toCharArray()) {
            dfs(x + 1, str + c, digits, phone, ans);
        }
    }
}