Unique Number of Occurrences
<p>Given an array of integers <code>arr</code>, return <code>true</code> <em>if the number of occurrences of each value in the array is <strong>unique</strong> or </em><code>false</code><em> otherwise</em>.</p>

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

<pre>
<strong>Input:</strong> arr = [1,2,2,1,1,3]
<strong>Output:</strong> true
<strong>Explanation:</strong>&nbsp;The value 1 has 3 occurrences, 2 has 2 and 3 has 1. No two values have the same number of occurrences.</pre>

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

<pre>
<strong>Input:</strong> arr = [1,2]
<strong>Output:</strong> false
</pre>

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

<pre>
<strong>Input:</strong> arr = [-3,0,1,-3,1,1,1,-3,10,0]
<strong>Output:</strong> true
</pre>

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

<ul>
	<li><code>1 &lt;= arr.length &lt;= 1000</code></li>
	<li><code>-1000 &lt;= arr[i] &lt;= 1000</code></li>
</ul>
Array; Hash Table
class Solution {
public:
    bool uniqueOccurrences(vector<int>& arr) {
        unordered_map<int, int> cnt;
        for (int& x : arr) {
            ++cnt[x];
        }
        unordered_set<int> vis;
        for (auto& [_, v] : cnt) {
            if (vis.count(v)) {
                return false;
            }
            vis.insert(v);
        }
        return true;
    }
};