2021.2.28刷题 回溯-电话字母组合

题目链接：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number
题目描述：

示例 1：
输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]

示例 2：
输入：digits = ""
输出：[]

示例 3：
输入：digits = "2"
输出：["a","b","c"]

题解：

class Solution {
public:
     unordered_map<char, string> phoneMap
     {
            {'2', "abc"},
            {'3', "def"},
            {'4', "ghi"},
            {'5', "jkl"},
            {'6', "mno"},
            {'7', "pqrs"},
            {'8', "tuv"},
            {'9', "wxyz"}
    };

    vector<string> result;
    string s;
    void backtracking(const string& digits, int index)
    {
        if(index == digits.size())
        {
            result.push_back(s);
            return;
        }
        string letters = phoneMap[digits[index]];
        for(int i = 0; i < letters.size(); i++)
        {
            s.push_back(letters[i]);
            backtracking(digits, index + 1);
            s.pop_back();
        }
    }
    vector<string> letterCombinations(string digits) {
        if(digits.size() == 0)
            return result;
        backtracking(digits, 0);
        return result;
    }
};