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 int[] secondGreaterElement(int[] nums) {
        int n = nums.length;
        int[] ans = new int[n];
        Arrays.fill(ans, -1);
        int[][] arr = new int[n][0];
        for (int i = 0; i < n; ++i) {
            arr[i] = new int[] {nums[i], i};
        }
        Arrays.sort(arr, (a, b) -> a[0] == b[0] ? a[1] - b[1] : b[0] - a[0]);
        TreeSet<Integer> ts = new TreeSet<>();
        for (int[] pair : arr) {
            int i = pair[1];
            Integer j = ts.higher(i);
            if (j != null && ts.higher(j) != null) {
                ans[i] = nums[ts.higher(j)];
            }
            ts.add(i);
        }
        return ans;
    }
}