0017. Letter Combinations of a Phone Number.cpp

// 17.✅ Letter Combinations of a Phone Number

class Solution
{
public:
    vector<string> letterCombinations(string digits)
    {
        vector<string> ans;
        if (digits.size() == 0)
            return ans;
        else if (digits.size() == 1)
        {
            if (digits[0] == '2')
                return {"a", "b", "c"};
            else if (digits[0] == '3')
                return {"d", "e", "f"};
            else if (digits[0] == '4')
                return {"g", "h", "i"};
            else if (digits[0] == '5')
                return {"j", "k", "l"};
            else if (digits[0] == '6')
                return {"m", "n", "o"};
            else if (digits[0] == '7')
                return {"p", "q", "r", "s"};
            else if (digits[0] == '8')
                return {"t", "u", "v"};
            else if (digits[0] == '9')
                return {"w", "x", "y", "z"};
        }
        else if (digits.size() > 1)
        {
            vector<string> str1 = letterCombinations(digits.substr(0, 1));
            vector<string> str2 = letterCombinations(digits.substr(1, digits.size() - 1));

            for (int i = 0; i < str1.size(); ++i)
            {
                for (int j = 0; j < str2.size(); ++j)
                {
                    ans.push_back(str1[i] + str2[j]);
                }
            }
        }
        return ans;
    }
};