LeetCode回溯系列（1）——第17题解法

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

  List<String> output = new ArrayList<String>();

  public void backtrack(String combination, String next_digits) {
    // if there is no more digits to check
    if (next_digits.length() == 0) {
        // the combination is done
        output.add(combination);
    }
    // if there are still digits to check
    else {
        // iterate over all letters which map 
        // the next available digit
        String digit = next_digits.substring(0, 1);
        String letters = phone.get(digit);
        for (int i = 0; i < letters.length(); i++) {
          String letter = phone.get(digit).substring(i, i + 1);
          // append the current letter to the combination
          // and proceed to the next digits
          backtrack(combination + letter, next_digits.substring(1));
        }
      }
  }

  public List<String> letterCombinations(String digits) {
    if (digits.length() != 0)
      backtrack("", digits);
    return output;
  }
}