<p>Given an integer array <code>nums</code>, return <em>the number of <strong>reverse pairs</strong> in the array</em>.</p>

<p>A <strong>reverse pair</strong> is a pair <code>(i, j)</code> where:</p>

<ul>
	<li><code>0 &lt;= i &lt; j &lt; nums.length</code> and</li>
	<li><code>nums[i] &gt; 2 * nums[j]</code>.</li>
</ul>

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

<pre>
<strong>Input:</strong> nums = [1,3,2,3,1]
<strong>Output:</strong> 2
<strong>Explanation:</strong> The reverse pairs are:
(1, 4) --&gt; nums[1] = 3, nums[4] = 1, 3 &gt; 2 * 1
(3, 4) --&gt; nums[3] = 3, nums[4] = 1, 3 &gt; 2 * 1
</pre>

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

<pre>
<strong>Input:</strong> nums = [2,4,3,5,1]
<strong>Output:</strong> 3
<strong>Explanation:</strong> The reverse pairs are:
(1, 4) --&gt; nums[1] = 4, nums[4] = 1, 4 &gt; 2 * 1
(2, 4) --&gt; nums[2] = 3, nums[4] = 1, 3 &gt; 2 * 1
(3, 4) --&gt; nums[3] = 5, nums[4] = 1, 5 &gt; 2 * 1
</pre>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 5 * 10<sup>4</sup></code></li>
	<li><code>-2<sup>31</sup> &lt;= nums[i] &lt;= 2<sup>31</sup> - 1</code></li>
</ul>

class Node:
    def __init__(self, val, cnt):
        self.val = val
        self.cnt = cnt
        self.l = None
        self.r = None
        self.more = 0

class Solution:
    def reversePairs(self, nums: List[int]) -> int:
        snums = sorted(set(nums))

        def build(l, r):
            m = (l + r) // 2
            node = Node(snums[m], 0)
            if m > l:
                node.l = build(l, m - 1)
            if m < r:
                node.r = build(m + 1, r)
            return node

        root = build(0, len(snums) - 1)

        def add(num, node):
            if node.val == num:
                node.cnt += 1
            elif node.val > num:
                if node.l:
                    add(num, node.l)
                else:
                    node.l = Node(num, 1)
            else:
                node.more += 1
                if node.r:
                    add(num, node.r)
                else:
                    node.r = Node(num, 1)

        def search(num, node):
            if node is None:
                return 0
            if node.val == num:
                return node.more
            if node.val > num:
                return node.more + node.cnt + search(num, node.l)
            else:
                return search(num, node.r)

        ans = 0
        for num in nums:
            ans += search(2 * num, root)
            add(num, root)
        return ans
30900