1. class Solution {
  2.     public List<String> letterCombinations(String digits) {
  3.         List<String> result = new ArrayList<>();
  4.  
  5.         if(digits.length() == 0)
  6.             return result;
  7.  
  8.         Map<Integer, String> map = new HashMap<>();
  9.         map.put(0, "");
  10.         map.put(1, "");
  11.         map.put(2, "abc");
  12.         map.put(3, "def");
  13.         map.put(4, "ghi");
  14.         map.put(5, "jkl");
  15.         map.put(6, "mno");
  16.         map.put(7, "pqrs");
  17.         map.put(8, "tuv");
  18.         map.put(9, "wxyz");
  19.  
  20.         String str = "";
  21.         findCombination(digits, 0, str, result, map);
  22.         return result;
  23.     }
  24.  
  25.     public void findCombination(String digits, int index, String str, List<String> result, Map<Integer, String> map)
  26.     {
  27.         if(index == digits.length())
  28.         {
  29.             result.add(str);
  30.             return;
  31.         }
  32.  
  33.         char digitChar = digits.charAt(index);
  34.         int digit = digitChar - '0';
  35.         String value = map.get(digit);
  36.         for(char ch : value.toCharArray())
  37.         {
  38.             findCombination(digits, index+1, str + ch, result, map);
  39.         }
  40.     }
  41. }
  42.  
  43. // digits = "34"
  44. // digitChar = '3' map.get('3')="def"