Phone Number Mnemonics

• Time : O(n * 4^n)
• Space : O(n * 4^n)

#include <vector>
using namespace std;

vector<string> phoneNumberMnemonics(string phoneNumber) {
    unordered_map<char, vector<char>> mp{
            {'1',{'1'}},
            {'2',{'a','b','c'}},
            {'3',{'d','e','f'}},
            {'4',{'g','h','i'}},
            {'5',{'j','k','l'}},
            {'6',{'m','n','o'}},
            {'7',{'p','q','r','s'}},
            {'8',{'t','u','v'}},
            {'9',{'w','x','y','z'}},
            {'0',{'0'}}
    };
    vector<string> res;
    res.push_back("");
    for(auto& ch : phoneNumber) {
        vector<string> tmp;
        for(auto& build : res) {
            for(auto& pad : mp[ch]) {
                tmp.push_back(build + string(1,pad));
            }
        }

        swap(res, tmp);
    }

    return res;
}