<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]
        items=[]
        for i in range(len(nums)):
            items.append((nums[i],i))
        self.mergeSort(items,ans,0,len(nums)-1)
        return ans[0]
    def mergeSort(self,items,ans,l,h):
        if l>=h:
            return
        mid=(l+h)//2
        self.mergeSort(items,ans,l,mid)
        self.mergeSort(items,ans,mid+1,h)
        self.merge(items,ans,l,mid,h)
    def merge(self,items,ans,l,mid,h):
        i=l
        j=mid+1
        while i<=mid and j<=h:
            if items[i][0]>2*items[j][0]:
                ans[0]+=(mid-i+1)
                j+=1
            else:
                i+=1
        i=l
        j=mid+1
        temp=[0]*(h-l+1)
        k=0

        while i<=mid and j<=h:
            if items[i][0]<items[j][0]:
                temp[k]=items[i]
                i+=1
                k+=1
            else:
                temp[k]=items[j]
                k+=1
                j+=1
        while i<=mid:
            temp[k]=items[i]
            i+=1
            k+=1
        while j<=h:
            temp[k]=items[j]
            k+=1
            j+=1
        k=0
        for i in range(l,h+1):
            items[i]=temp[k]
            k+=1


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