Replace Elements with Greatest Element on Right Side
<p>Given an array <code>arr</code>,&nbsp;replace every element in that array with the greatest element among the elements to its&nbsp;right, and replace the last element with <code>-1</code>.</p>

<p>After doing so, return the array.</p>

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

<pre>
<strong>Input:</strong> arr = [17,18,5,4,6,1]
<strong>Output:</strong> [18,6,6,6,1,-1]
<strong>Explanation:</strong> 
- index 0 --&gt; the greatest element to the right of index 0 is index 1 (18).
- index 1 --&gt; the greatest element to the right of index 1 is index 4 (6).
- index 2 --&gt; the greatest element to the right of index 2 is index 4 (6).
- index 3 --&gt; the greatest element to the right of index 3 is index 4 (6).
- index 4 --&gt; the greatest element to the right of index 4 is index 5 (1).
- index 5 --&gt; there are no elements to the right of index 5, so we put -1.
</pre>

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

<pre>
<strong>Input:</strong> arr = [400]
<strong>Output:</strong> [-1]
<strong>Explanation:</strong> There are no elements to the right of index 0.
</pre>

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

<ul>
	<li><code>1 &lt;= arr.length &lt;= 10<sup>4</sup></code></li>
	<li><code>1 &lt;= arr[i] &lt;= 10<sup>5</sup></code></li>
</ul>
Array
class Solution {
public:
    vector<int> replaceElements(vector<int>& arr) {
        for (int i = arr.size() - 1, mx = -1; ~i; --i) {
            int x = arr[i];
            arr[i] = mx;
            mx = max(mx, x);
        }
        return arr;
    }
};
