<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(), digits = 0, letters = 0;
	    stack<char> alpha;
        stack<char> num;
	    for (char c : s) 
        {   if (isalpha(c)) 
            {   alpha.push(c);
                letters++;
            }
		    if (isdigit(c))
            {   num.push(c);
                digits++;
            }
	    }
	    if (abs(letters - digits) > 1) 
            return "";

	    string result;
        while ( !alpha.empty() && !num.empty() ) 
        {   char d = num.top();
            char a = alpha.top();
            num.pop();
            alpha.pop();
            if (alpha.size() < num.size()) {
                result+=d;
                result+=a;     
            }
            else 
            {   result+=a;
                result+=d; 
            }
        }
        if(!alpha.empty())
            result+=alpha.top();
        else if(!num.empty())
            result+=num.top();
        
        return result;
    }
};
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