#include<iostream>
#include<vector>
#include<string>
#include <unordered_map>

using namespace std;

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> combinations;
        if (digits.empty()) {
            return combinations;
        }
        unordered_map<char, string> phoneMap{{'2', "abc"},{'3', "def"},{'4', "ghi"},{'5', "jkl"},{'6', "mno"},
        {'7', "pqrs"},{'8', "tuv"},{'9', "wxyz"}};
        string combination;
        backtrack(combinations, phoneMap, digits, 0, combination);
        return combinations;
    }

    void backtrack(vector<string>& combinations, const unordered_map<char, string>& phoneMap, const string& digits, int index, string& combination) {
        if (index == digits.length()) {
            combinations.push_back(combination);
        } else {
            char digit = digits[index];
            const string& letters = phoneMap.at(digit);
            for (const char& letter: letters) {
                combination.push_back(letter);
                backtrack(combinations, phoneMap, digits, index + 1, combination);
                combination.pop_back();
            }
        }
    }
};

int main() {
    Solution S;
    vector<string> ans;
    string digits=to_string(23);
    ans=S.letterCombinations(digits);
    for (auto str:ans){
        cout<<str<<","<<flush;
    }
    return 0;
}
结果：ad,ae,af,bd,be,bf,cd,ce,cf,