Letter Combinations of a Phone Number @LeetCode

package Level3;

import java.util.ArrayList;

/**
 * Letter Combinations of a Phone Number 
 * 
 *  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.
https://www.cppentry.com/upload_files/article/76/1_mftcv__.png


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 S17 {

	public static void main(String[] args) {
		System.out.println(letterCombinations("23"));
	}

	public static ArrayList





 letterCombinations(String digits) {
		String[] ss = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
		
		ArrayList ret = new ArrayList();
		rec(ret, digits.length(), ss, digits, new StringBuffer());
		return ret;
    }
	
	public static void rec(ArrayList ret, int remain, String[] ss, String digits, StringBuffer sb){
		// 说明对digits遍历结束
		if(remain == 0){
			ret.add(sb.toString());
			return;
		}
		
		String s = ss[digits.charAt(0)-'0'];		// 得到digits[0]对应的string s
		// DFS
		for(int i=0; i