LetterCombinations.java

package leetcode;

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

/**
 * Created by nikoo28 on 7/18/19 3:21 AM
 */

class LetterCombinations {

  public static List<String> letterCombinations(String digits) {
    String[] digitletter = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    List<String> result = new ArrayList<>();

    if (digits.length()==0) return result;

    result.add("");
    for (int i=0; i<digits.length(); i++)
      result = combine(digitletter[digits.charAt(i)-'0'],result);

    return result;
  }

  private static List<String> combine(String digit, List<String> l) {
    List<String> result = new ArrayList<>();

    for (int i=0; i<digit.length(); i++)
      for (String x : l)
        result.add(x+digit.charAt(i));

    return result;
  }

}