Better Compression of String
<p>You are given a string <code>compressed</code> representing a compressed version of a string. The format is a character followed by its frequency. For example, <code>&quot;a3b1a1c2&quot;</code> is a compressed version of the string <code>&quot;aaabacc&quot;</code>.</p>

<p>We seek a <strong>better compression</strong> with the following conditions:</p>

<ol>
	<li>Each character should appear <strong>only once</strong> in the compressed version.</li>
	<li>The characters should be in <strong>alphabetical order</strong>.</li>
</ol>

<p>Return the <em>better compression</em> of <code>compressed</code>.</p>

<p><strong>Note:</strong> In the better version of compression, the order of letters may change, which is acceptable.</p>

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

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">compressed = &quot;a3c9b2c1&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">&quot;a3b2c10&quot;</span></p>

<p><strong>Explanation:</strong></p>

<p>Characters &quot;a&quot; and &quot;b&quot; appear only once in the input, but &quot;c&quot; appears twice, once with a size of 9 and once with a size of 1.</p>

<p>Hence, in the resulting string, it should have a size of 10.</p>
</div>

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

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">compressed = &quot;c2b3a1&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">&quot;a1b3c2&quot;</span></p>
</div>

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

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">compressed = &quot;a2b4c1&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">&quot;a2b4c1&quot;</span></p>
</div>

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

<ul>
	<li><code>1 &lt;= compressed.length &lt;= 6 * 10<sup>4</sup></code></li>
	<li><code>compressed</code> consists only of lowercase English letters and digits.</li>
	<li><code>compressed</code> is a valid compression, i.e., each character is followed by its frequency.</li>
	<li>Frequencies are in the range <code>[1, 10<sup>4</sup>]</code> and have no leading zeroes.</li>
</ul>
Hash Table; String; Counting; Sorting
class Solution {
public:
    string betterCompression(string compressed) {
        map<char, int> cnt;
        int i = 0;
        int n = compressed.length();
        while (i < n) {
            char c = compressed[i];
            int j = i + 1;
            int x = 0;
            while (j < n && isdigit(compressed[j])) {
                x = x * 10 + (compressed[j] - '0');
                j++;
            }
            cnt[c] += x;
            i = j;
        }
        stringstream ans;
        for (const auto& entry : cnt) {
            ans << entry.first << entry.second;
        }
        return ans.str();
    }
};