Counting Elements
<p>Given an integer array <code>arr</code>, count how many elements <code>x</code> there are, such that <code>x + 1</code> is also in <code>arr</code>. If there are duplicates in <code>arr</code>, count them separately.</p>

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

<pre>
<strong>Input:</strong> arr = [1,2,3]
<strong>Output:</strong> 2
<strong>Explanation:</strong> 1 and 2 are counted cause 2 and 3 are in arr.
</pre>

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

<pre>
<strong>Input:</strong> arr = [1,1,3,3,5,5,7,7]
<strong>Output:</strong> 0
<strong>Explanation:</strong> No numbers are counted, cause there is no 2, 4, 6, or 8 in arr.
</pre>

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

<ul>
	<li><code>1 &lt;= arr.length &lt;= 1000</code></li>
	<li><code>0 &lt;= arr[i] &lt;= 1000</code></li>
</ul>
Array; Hash Table
class Solution {
    public int countElements(int[] arr) {
        int[] cnt = new int[1001];
        for (int x : arr) {
            ++cnt[x];
        }
        int ans = 0;
        for (int x = 0; x < 1000; ++x) {
            if (cnt[x + 1] > 0) {
                ans += cnt[x];
            }
        }
        return ans;
    }
}