<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:
    void backtrack(vector<string>& res, vector<vector<char>> element, string str, int i){
        if (str.size() == element.size()){
            res.push_back(str);
            return;
        }

        for (int j = 0; j < element[i].size(); j++){
            str+=element[i][j];
            backtrack(res, element, str, i + 1);
            str.pop_back();
        }
    }
    vector<string> letterCombinations(string digits) {
        if (digits.size() == 0) return {};
        unordered_map<char, vector<char>> map;
        map['2'] = {'a', 'b', 'c'};
        map['3'] = {'d', 'e', 'f'};
        map['4'] = {'g', 'h', 'i'};
        map['5'] = {'j', 'k','l'};
        map['6'] = {'m', 'n', 'o'};
        map['7'] = {'p', 'q', 'r', 's'};
        map['8'] = {'t', 'u', 'v'};
        map['9'] = {'w','x','y','z'};

        vector<string> res;
        vector<vector<char>> element;
        for (char ch : digits){
            element.push_back(map[ch]);
        }

        backtrack(res, element,"", 0);

        return res;
        


    }
};
9000