<p>Given a string containing digits from <code>2-9</code> inclusive, return all possible letter combinations that the number could represent. Return the answer in <strong>any order</strong>.</p>

<p>A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.</p>
<img alt="" src="https://assets.leetcode.com/uploads/2022/03/15/1200px-telephone-keypad2svg.png" style="width: 300px; height: 243px;" />
<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> digits = &quot;23&quot;
<strong>Output:</strong> [&quot;ad&quot;,&quot;ae&quot;,&quot;af&quot;,&quot;bd&quot;,&quot;be&quot;,&quot;bf&quot;,&quot;cd&quot;,&quot;ce&quot;,&quot;cf&quot;]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> digits = &quot;&quot;
<strong>Output:</strong> []
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> digits = &quot;2&quot;
<strong>Output:</strong> [&quot;a&quot;,&quot;b&quot;,&quot;c&quot;]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= digits.length &lt;= 4</code></li>
	<li><code>digits[i]</code> is a digit in the range <code>[&#39;2&#39;, &#39;9&#39;]</code>.</li>
</ul>

class Solution {
public:
    vector<string> FilterLetters(string digits) {
        unordered_map<char, string> mp = {
            {'2', "abc"}, {'3', "def"},  {'4', "ghi"}, {'5', "jkl"},
            {'6', "mno"}, {'7', "pqrs"}, {'8', "tuv"}, {'9', "wxyz"}};

        vector<string> ret_strings;
        for (char digit : digits) {
            ret_strings.push_back(mp[digit]);
        }
        return ret_strings;
    }

    vector<string> GetCombinations(vector<string> strings) {
        vector<string> ret_combination;
        if (strings.size() == 0) {
            return ret_combination;
        }
        if (strings.size() == 1) {
            for(char c:strings[0]) {
                string char_string(1, c);
                ret_combination.push_back(char_string);
            }
            return ret_combination;
        }

        for(char c:strings[0]) {
            std::vector<string> rest_of_vec(strings.begin() + 1, strings.end());
            auto combinations = GetCombinations(rest_of_vec);
            for (string combination : combinations) {
                string char_string(1, c);
                ret_combination.push_back(char_string + combination);
            }
        }
        return ret_combination;
    }

    vector<string> letterCombinations(string digits) {
        vector<string> filtered_strings = FilterLetters(digits);

        return GetCombinations(filtered_strings);
    }
};
9000