<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:
        count=0
        def countpairs(low,mid,high):
            nonlocal nums, count
            right=mid+1
            for i in range(low,mid+1):
                while right<=high and nums[i]>2*nums[right]:
                    right+=1
                count+=(right-(mid+1))
                # for j in range(mid+1,high+1):
                #     if nums[i]>2*nums[j]:
                #         count+=1
                #     else:
                #         break
        def merge(low,mid,high):
            nonlocal nums
            left=low
            right=mid+1
            temp=[]
            while left<=mid and right<=high:
                if nums[left]<=nums[right]:
                    temp.append(nums[left])
                    left+=1
                else:
                    temp.append(nums[right])
                    right+=1
            while left<=mid:
                temp.append(nums[left])
                left+=1
            while right<=high:
                temp.append(nums[right])
                right+=1
            for i in range(len(temp)):
                nums[low+i]=temp[i]

        def mergesort(low,high):
            nonlocal nums
            if low<high:
                mid=(low+high)//2
                mergesort(low,mid)
                mergesort(mid+1,high)
                countpairs(low,mid,high)
                merge(low,mid,high)

        mergesort(0,len(nums)-1)
        return count
        
29500