<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 Solution:
    def reversePairs(self, nums: List[int]) -> int:
        ans = 0
        def merge(l, r) -> List[int]:
            nonlocal ans
            if l == r:
                return [nums[l]]
            m = (l+r) // 2

            l_arr = merge(l, m)
            r_arr = merge(m+1, r)

            i,j = 0,0
            while i < len(l_arr) and j < len(r_arr):
                if l_arr[i] > (2 * r_arr[j]):
                    ans += len(l_arr) - i
                    j += 1
                else:
                    i += 1

            i,j = 0,0
            ret = []

            while i < len(l_arr) and j < len(r_arr):
                if l_arr[i] > r_arr[j]:
                    ret.append(r_arr[j])
                    j += 1
                else:
                    ret.append(l_arr[i])
                    i += 1

            while i < len(l_arr):
                ret.append(l_arr[i])
                i += 1
            
            while j < len(r_arr):
                ret.append(r_arr[j])
                j += 1
            
            return ret

        merge(0, len(nums) - 1)
        return ans

30600