<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:
    map<int,string> buttons;
    Solution(){
        buttons[2]="abc";
        buttons[3]="def";
        buttons[4]="ghi";
        buttons[5]="jkl";
        buttons[6]="mno";
        buttons[7]="pqrs";
        buttons[8]="tuv";
        buttons[9]="wxyz";
    }

    void solve(vector<string>& out, string digits,int index,string comb){
        if(index==digits.size()) {
            out.push_back(comb);
        }
        int num = (int)digits[index] - 48;

        for(int i=0;i<buttons[num].size();i++){
            comb+=buttons[num][i];
            solve(out,digits,index+1,comb);
            comb.pop_back();
        }
    }

    vector<string> letterCombinations(string digits) {
        if(!digits.size()) return {};

        vector<string> out;
        string comb="";
        solve(out,digits,0,comb);

        return out;
    }
};

9100