LetterPhoneNumber.java

/* (C) 2024 YourCompanyName */
package backtracking;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by gouthamvidyapradhan on 09/03/2017. Given a digit string, return all possible letter
 * combinations that the number could represent.
 *
 * <p>A mapping of digit to letters (just like on the telephone buttons) is given below. 1 2(abc)
 * 3(def) 4(ghi) 5(jkl) 6(mno) 7(pqrs) 8(tuv) 9(wxyz)
 *
 * <p>
 *
 * <p>Input:Digit string "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]. Note:
 * Although the above answer is in lexicographical order, your answer could be in any order you
 * want.
 */
public class LetterPhoneNumber {
  private String[] NUMBER_ALPHA = {
    "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"
  };

  /**
   * Main method
   *
   * @param args
   * @throws Exception
   */
  public static void main(String[] args) throws Exception {
    List<String> result = new LetterPhoneNumber().letterCombinations("23");
    result.forEach(System.out::println);
  }

  private List<String> letterCombinations(String digits) {
    if (digits == null || digits.isEmpty() || digits.contains("1") || digits.contains("0"))
      return new ArrayList<>();
    List<String> prev = new ArrayList<>();
    prev.add("");
    for (int i = digits.length() - 1; i >= 0; i--) {
      String str = NUMBER_ALPHA[Integer.parseInt(String.valueOf(digits.charAt(i)))];
      List<String> newList = new ArrayList<>();
      for (int j = 0, l = str.length(); j < l; j++) {
        for (String s : prev) {
          s = str.charAt(j) + s;
          newList.add(s);
        }
      }
      prev = newList;
    }
    return prev;
  }
}