Letter Combinations of a Phone Number

Problem

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"].

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Idea

dfs暴力搜索，注意递归的参数设置。

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Solution

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