## 题目地址

https://leetcode.com/problems/letter-combinations-of-a-phone-number/

## 题目描述

Given a string containing digits from 2-9inclusive, 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. Note that 1 does not map to any letters.

Example:

Input: "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.

## 代码

class Solution {
public:
vector<string> letterCombinations(string digits) {
if (digits.empty()) return {};
vector<string> res;
vector<string> dict{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
letterCombinationsDFS(digits, dict, 0, "", res);
return res;
}
void letterCombinationsDFS(string& digits, vector<string>& dict, int level, string out, vector<string>& res) {
if (level == digits.size()) {res.push_back(out); return;}
string str = dict[digits[level] - '0'];
for (int i = 0; i < str.size(); ++i) {
letterCombinationsDFS(digits, dict, level + 1, out + str[i], res);
}
}
};

class Solution {
public:
vector<string> letterCombinations(string digits) {
if (digits.empty()) return {};
vector<string> res{""};
vector<string> dict{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
for (int i = 0; i < digits.size(); ++i) {
vector<string> t;
string str = dict[digits[i] - '0'];
for (int j = 0; j < str.size(); ++j) {
for (string s : res) t.push_back(s + str[j]);
}
res = t;
}
return res;
}
};

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