<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 mapping(int i, string& digits, vector<string>& dict, string comb, vector<string>& ans)
    {
        if (i==digits.size())
        {
            if (comb.size()>0)
                ans.push_back(comb);
            return;
        }

        for (auto d: dict[digits[i]-'0'])
            mapping(i+1, digits, dict, comb+d, ans);
    }
    vector<string> letterCombinations(string digits)
    {
        vector<string> dict = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        vector<string> ans;
        string comb="";
        mapping(0, digits, dict, comb, ans);
        return ans;  
    }
};
8100