Leetcode 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"].

Difficulty: Medium

public class Solution {

public List<String> letterCombinations(String digits) {
HashMap<Integer, String> hm = new HashMap<Integer, String>();
List<String> list = new ArrayList<String>();
if(digits == null)
return list;
hm.put(2, "abc");
hm.put(3, "def");
hm.put(4, "ghi");
hm.put(5, "jkl");
hm.put(6, "mno");
hm.put(7, "pqrs");
hm.put(8, "tuv");
hm.put(9, "wxyz");
hm.put(0, "");

for(int i = 0; i < digits.length(); i++){
String s = hm.get(digits.charAt(i) - '0');
int len = list.size();
for(int j = 0; j < s.length(); j++){
if(len == 0){
list.add("" + s.charAt(j));
}
else{
for(int k = 0; k < len; k++){
list.add(list.get(k) + s.charAt(j));
}
}
}
for(int k = 0; k < len; k++){
list.remove(0);
}
}
return list;

}
}