<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> letterCombinations(string digits) {
    if (digits.empty())
      return {};

    vector<string> ans;

    dfs(digits, 0, "", ans);
    return ans;
  }

 private:
  const vector<string> digitToLetters{"",    "",    "abc",  "def", "ghi",
                                      "jkl", "mno", "pqrs", "tuv", "wxyz"};

  void dfs(const string& digits, int i, string&& path, vector<string>& ans) {
    if (i == digits.length()) {
      ans.push_back(path);
      return;
    }

    for (const char letter : digitToLetters[digits[i] - '0']) {
      path.push_back(letter);
      dfs(digits, i + 1, std::move(path), ans);
      path.pop_back();
    }
  }
};
8100