<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) {
        int n = s.size();
        vector<char> num;
        vector<char> arr;

        // Separate digits and letters into two vectors
        for (int i = 0; i < n; i++) {
            if (isdigit(s[i])) {
                num.push_back(s[i]);
            } else {
                arr.push_back(s[i]);
            }
        }

        // If the difference in sizes is greater than 1, return empty string
        if (abs((int)num.size() - (int)arr.size()) > 1) {
            return "";
        }

        // Initialize result string
        string res = "";
        int i = 0, j = 0;

        // Decide whether to start with a digit or letter
        bool startWithDigit = num.size() >= arr.size();

        // Construct the result by alternating between digits and letters
        while (i < num.size() || j < arr.size()) {
            if (startWithDigit && i < num.size()) {
                res += num[i++];
            } else if (!startWithDigit && j < arr.size()) {
                res += arr[j++];
            }
            // Alternate the starting character
            startWithDigit = !startWithDigit;
        }

        return res;
    }
};

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