Leetcode原题电话号码的字母组合的两种解法【BFS-DFS】

来源：LeetCode 第17题【公众号：数据结构和算法】

给定一个仅包含数字2-9的字符串，返回所有它能表示的字母组合。答案可以按任意顺序返回。给出数字到字母的映射如下（与电话按键相同）。注意1不对应任何字母。

示例1：
输入：d i g i t s = " 2 3 " 
输出：[" a d "," a e "," a f"," b d "," b e "," b f"," cd "," ce "," c f"]
 
 
示例2：
输入：d i g i t s = "" 
输出：[ ] 
 
 
示例3：
输入：d i g i t s = " 2 " 
输出：[" a "," b "," c "] 

提示：

●  0<=digits.length<=4

●  digits[]是范围['