<p>You are given an alphanumeric string <code>s</code>. (<strong>Alphanumeric string</strong> is a string consisting of lowercase English letters and digits).</p>

<p>You have to find a permutation of the string where no letter is followed by another letter and no digit is followed by another digit. That is, no two adjacent characters have the same type.</p>

<p>Return <em>the reformatted string</em> or return <strong>an empty string</strong> if it is impossible to reformat the string.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;a0b1c2&quot;
<strong>Output:</strong> &quot;0a1b2c&quot;
<strong>Explanation:</strong> No two adjacent characters have the same type in &quot;0a1b2c&quot;. &quot;a0b1c2&quot;, &quot;0a1b2c&quot;, &quot;0c2a1b&quot; are also valid permutations.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;leetcode&quot;
<strong>Output:</strong> &quot;&quot;
<strong>Explanation:</strong> &quot;leetcode&quot; has only characters so we cannot separate them by digits.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;1229857369&quot;
<strong>Output:</strong> &quot;&quot;
<strong>Explanation:</strong> &quot;1229857369&quot; has only digits so we cannot separate them by characters.
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 500</code></li>
	<li><code>s</code> consists of only lowercase English letters and/or digits.</li>
</ul>

class Solution {
 public:
  string reformat(string s) {
    const int countAlpha =
        ranges::count_if(s, [](char c) { return isalpha(c); });
    const int countDigit = s.size() - countAlpha;
    if (abs(countAlpha - countDigit) >= 2)
      return "";

    // Initialize the starting index. e.g. "a0a0a" or "0a0a0".
    const int alphaStartingIndex = countAlpha >= countDigit ? 0 : 1;

    // Place all alphas in the indices 0, 2, 4, 6, ... or 1, 3, 5, 7, ....
    for (int i = 0, j = alphaStartingIndex; i < s.length(); ++i) {
      if (i < j && i % 2 == alphaStartingIndex)
        continue;  // The `s[i]` has been set.
      if (!isalpha(s[i]))
        continue;
      swap(s[i], s[j]);
      j += 2;
      --i;
    }

    return s;
  }
};
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