Next Greater Element IV
<p>You are given a <strong>0-indexed</strong> array of non-negative integers <code>nums</code>. For each integer in <code>nums</code>, you must find its respective <strong>second greater</strong> integer.</p>

<p>The <strong>second greater</strong> integer of <code>nums[i]</code> is <code>nums[j]</code> such that:</p>

<ul>
	<li><code>j &gt; i</code></li>
	<li><code>nums[j] &gt; nums[i]</code></li>
	<li>There exists <strong>exactly one</strong> index <code>k</code> such that <code>nums[k] &gt; nums[i]</code> and <code>i &lt; k &lt; j</code>.</li>
</ul>

<p>If there is no such <code>nums[j]</code>, the second greater integer is considered to be <code>-1</code>.</p>

<ul>
	<li>For example, in the array <code>[1, 2, 4, 3]</code>, the second greater integer of <code>1</code> is <code>4</code>, <code>2</code> is <code>3</code>,&nbsp;and that of <code>3</code> and <code>4</code> is <code>-1</code>.</li>
</ul>

<p>Return<em> an integer array </em><code>answer</code><em>, where </em><code>answer[i]</code><em> is the second greater integer of </em><code>nums[i]</code><em>.</em></p>

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

<pre>
<strong>Input:</strong> nums = [2,4,0,9,6]
<strong>Output:</strong> [9,6,6,-1,-1]
<strong>Explanation:</strong>
0th index: 4 is the first integer greater than 2, and 9 is the second integer greater than 2, to the right of 2.
1st index: 9 is the first, and 6 is the second integer greater than 4, to the right of 4.
2nd index: 9 is the first, and 6 is the second integer greater than 0, to the right of 0.
3rd index: There is no integer greater than 9 to its right, so the second greater integer is considered to be -1.
4th index: There is no integer greater than 6 to its right, so the second greater integer is considered to be -1.
Thus, we return [9,6,6,-1,-1].
</pre>

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

<pre>
<strong>Input:</strong> nums = [3,3]
<strong>Output:</strong> [-1,-1]
<strong>Explanation:</strong>
We return [-1,-1] since neither integer has any integer greater than it.
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>0 &lt;= nums[i] &lt;= 10<sup>9</sup></code></li>
</ul>
Stack; Array; Binary Search; Sorting; Monotonic Stack; Heap (Priority Queue)
class Solution {
public:
    vector<int> secondGreaterElement(vector<int>& nums) {
        int n = nums.size();
        vector<int> ans(n, -1);
        vector<pair<int, int>> arr(n);
        for (int i = 0; i < n; ++i) {
            arr[i] = {-nums[i], i};
        }
        sort(arr.begin(), arr.end());
        set<int> ts;
        for (auto& [_, i] : arr) {
            auto it = ts.upper_bound(i);
            if (it != ts.end() && ts.upper_bound(*it) != ts.end()) {
                ans[i] = nums[*ts.upper_bound(*it)];
            }
            ts.insert(i);
        }
        return ans;
    }
};