<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) {
        unordered_map<char, int> count;
        int alpha_cnt = 0;
        for (const auto& c : s) {
            ++count[c];
            if (isalpha(c)) {
                ++alpha_cnt;
            }
        }
        if (abs(int(s.length()) - 2 * alpha_cnt) > 1) {
            return "";
        }
        
        char a = 'a', a_end = 'z';
        char b = '0', b_end = '9';
        if (alpha_cnt < s.length() - alpha_cnt) {
            swap(a, b);
            swap(a_end, b_end);
        }
        string result;
        while (result.length() < s.length()) {
            result.push_back(a = next_char(a, a_end, &count));
            result.push_back(b = next_char(b, b_end, &count));
        }
        if (result.back() == '\0') {
            result.pop_back();
        }
        return result;
    }

private:
    char next_char(char start, char end, unordered_map<char, int> *count) {
        for (char c = start; c <= end; ++c) {
            if ((*count)[c]) {
                --(*count)[c];
                return c;
            }
        }
        return '\0';
    }
};
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