<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:
        n=len(nums)

        def merge(arr,left,mid,right):
            temp=[]
            cnt=0
            low=left
            high=mid+1
            j=high
            for i in range(left , mid+1):
                while j<=right and arr[i]>2*arr[j]:
                    j+=1
                cnt+=j-high
            while low<=mid and high<=right:
                if arr[low]<=arr[high]:
                    temp.append(arr[low])
                    low+=1
                else:
                    temp.append(arr[high])
                    high+=1
            while low<=mid:
                temp.append(arr[low])
                low+=1
            while high<=right:
                temp.append(arr[high])
                high+=1
            for i in range(left,right+1):
                arr[i]=temp[i-left]
            return cnt


        def mergesort(arr,left,right):
            cnt=0
            if left>=right:
                return cnt
            mid=(left+right)//2
            cnt+=mergesort(arr,left,mid)
            cnt+=mergesort(arr,mid+1,right)
            cnt+=merge(arr,left,mid,right)
            return cnt
        
        
        return mergesort(nums,0,n-1)
    
    
23800