<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:
    global_answer = 0
    def merge(self,sublist,l,mid,h):
        temp = [0]*(h-l+1)
        k=0

        i=l
        j=mid+1

        while i<=mid and j<=h:
            if sublist[i]>2*sublist[j]:
                Solution.global_answer+=(mid-i+1)
                j+=1
            else:
                i+=1

        #now merging
        i=l
        j= mid+1
        k=0

        while i<=mid and j<=h:
            if sublist[i]<sublist[j]:
                temp[k]=sublist[i]
                i+=1
            else:
                temp[k]=sublist[j]
                j+=1
            k+=1

        while i<=mid:
            temp[k]=sublist[i]
            k+=1
            i+=1
        
        while j<=h:
            temp[k]=sublist[j]
            k+=1
            j+=1

        for ptr in range(h-l+1):
            sublist[l+ptr] = temp[ptr] 
    
    def merge_splitter(self,sublist,l,h):
        if l<h: #atleast 2 elements per sublist

            mid = (l+h)//2
            self.merge_splitter(sublist,l,mid)
            self.merge_splitter(sublist,mid+1,h)
            
            self.merge(sublist,l,mid,h)




    def reversePairs(self, nums: List[int]) -> int:
        Solution.global_answer=0
        self.merge_splitter(nums, 0, len(nums)-1)
        return Solution.global_answer


        
23900