Longest Happy String
<p>A string <code>s</code> is called <strong>happy</strong> if it satisfies the following conditions:</p>

<ul>
	<li><code>s</code> only contains the letters <code>&#39;a&#39;</code>, <code>&#39;b&#39;</code>, and <code>&#39;c&#39;</code>.</li>
	<li><code>s</code> does not contain any of <code>&quot;aaa&quot;</code>, <code>&quot;bbb&quot;</code>, or <code>&quot;ccc&quot;</code> as a substring.</li>
	<li><code>s</code> contains <strong>at most</strong> <code>a</code> occurrences of the letter <code>&#39;a&#39;</code>.</li>
	<li><code>s</code> contains <strong>at most</strong> <code>b</code> occurrences of the letter <code>&#39;b&#39;</code>.</li>
	<li><code>s</code> contains <strong>at most</strong> <code>c</code> occurrences of the letter <code>&#39;c&#39;</code>.</li>
</ul>

<p>Given three integers <code>a</code>, <code>b</code>, and <code>c</code>, return <em>the <strong>longest possible happy </strong>string</em>. If there are multiple longest happy strings, return <em>any of them</em>. If there is no such string, return <em>the empty string </em><code>&quot;&quot;</code>.</p>

<p>A <strong>substring</strong> is a contiguous sequence of characters within a string.</p>

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

<pre>
<strong>Input:</strong> a = 1, b = 1, c = 7
<strong>Output:</strong> &quot;ccaccbcc&quot;
<strong>Explanation:</strong> &quot;ccbccacc&quot; would also be a correct answer.
</pre>

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

<pre>
<strong>Input:</strong> a = 7, b = 1, c = 0
<strong>Output:</strong> &quot;aabaa&quot;
<strong>Explanation:</strong> It is the only correct answer in this case.
</pre>

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

<ul>
	<li><code>0 &lt;= a, b, c &lt;= 100</code></li>
	<li><code>a + b + c &gt; 0</code></li>
</ul>
Greedy; String; Heap (Priority Queue)
class Solution {
public:
    string longestDiverseString(int a, int b, int c) {
        using pci = pair<char, int>;
        auto cmp = [](pci x, pci y) { return x.second < y.second; };
        priority_queue<pci, vector<pci>, decltype(cmp)> pq(cmp);

        if (a > 0) pq.push({'a', a});
        if (b > 0) pq.push({'b', b});
        if (c > 0) pq.push({'c', c});

        string ans;
        while (!pq.empty()) {
            pci cur = pq.top();
            pq.pop();
            int n = ans.size();
            if (n >= 2 && ans[n - 1] == cur.first && ans[n - 2] == cur.first) {
                if (pq.empty()) break;
                pci nxt = pq.top();
                pq.pop();
                ans.push_back(nxt.first);
                if (--nxt.second > 0) {
                    pq.push(nxt);
                }
                pq.push(cur);
            } else {
                ans.push_back(cur.first);
                if (--cur.second > 0) {
                    pq.push(cur);
                }
            }
        }

        return ans;
    }
};