[LeetCode 17] Letter Combinations of a Phone Number

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

/**
 * 
		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"].
		Note:
		Although the above answer is in lexicographical order, your answer could be in any order you want.
 *
 */

public class LetterCombinationsOfAPhoneNumber {
	
	public static String[] mapping = new String[] {"", "", "abc", "def", 
												   "ghi", "jkl", "mno", 
												   "pqrs", "tuv", "wxyz"};
	
    //解法一：递归法
//  25 / 25 test cases passed.
//  Status: Accepted
//  Runtime: 208 ms
//  Submitted: 0 minutes ago
	

  static List<String> letterCombinations1(String digits) {
		List<String> combinations = new ArrayList<String>();
		dfs(digits, "", combinations);
      return 	combinations;
  }
  static void dfs(String digits, String path, List<String> combinations) {
  	if(digits.length() == 0) return;
  	else if(digits.length() == 1) {
  		for(Character letter : mapping[digits.charAt(0) - '0'].toCharArray()) {
  			combinations.add(path + letter); 
  		}
  	} else {
  		for(Character letter : mapping[digits.charAt(0) - '0'].toCharArray()) {
  			dfs(digits.substring(1), path + letter, combinations);
  		}
  	}
  }
	
	//解法二：遍历法
  
//  25 / 25 test cases passed.
//  Status: Accepted
//  Runtime: 211 ms
//  Submitted: 0 minutes ago

    static List<String> letterCombinations(String digits) {
		List<String> combinations = new ArrayList<String>();
		if(digits.length() == 0) return combinations;
		for (Character c : digits.toCharArray()) {
			String letters = mapping[c - '0'];
			if(combinations.size() == 0) {
				for (Character letter : letters.toCharArray()) {
					combinations.add(letter + "");
				}
			} else {
				int length = combinations.size();
				for (Character letter : letters.toCharArray()) {
					for(int i = 0; i < length; i ++) {
						String before = combinations.get(i);
						combinations.add(before + letter);
					}	
				}
				for (int i = 0; i < length; i++) {
					combinations.remove(0);
				}
			}
		}
        return combinations;
    }
    

	public static void main(String[] args) {
		System.out.println(Arrays.toString(letterCombinations("234").toArray()));
	}

}