<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 {
    private:
    int mergesort(vector<int>& arr,int left,int right){
        if (left>=right){
            return 0;
        }
        int mid = left + (right-left)/2;
        int ans1 = mergesort(arr,left,mid);
        int ans2 = mergesort(arr,mid+1,right);
        vector<int> temp1;
        vector<int> temp2;
        int ctr = 0;
        int sz1 = mid - left + 1;
        int sz2 = right - mid;
        for (int i=left;i<=mid;i++){
            temp1.push_back(arr[i]);
        }
        for (int i=mid+1;i<=right;i++){
            temp2.push_back(arr[i]);
        }
        int p = 0;
        int l = 0;
        while (p<sz1){
            while (l<sz2 && temp1[p]>2*(long)temp2[l]) l++;
            ctr += l;
            p++;
        }
        int i=0,j=0;
        int k = left;
        while (i<sz1 && j<sz2){
            if (temp1[i]<temp2[j]){
                arr[k] = temp1[i];
                i++;
            }else{
                arr[k] = temp2[j];
                j++;
            }
            k++;
        }
        while (i<sz1){
            arr[k] = temp1[i];
            k++;
            i++;
        }
        while (j<sz2){
            arr[k] = temp2[j];
            k++;
            j++;
        }
        return ans1 + ans2 + ctr;
    }

public:
    int reversePairs(vector<int>& nums) {
        int n = nums.size();
        vector<int> temp(nums.begin(),nums.end());
        return mergesort(temp,0,n-1);
    }
};
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