<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 countPairs(vector<long long>& temp1,int len1,vector<long long>& temp2,int len2){
        int count=0;
        for(int i=0,j=0;i<len1;i++){
            while(j<len2 && temp1[i]>2*temp2[j]) j++;
            count+=j;
        }
        return count;
    }
    int merge(vector<int>& nums,int low,int mid,int high){
        int len1=mid-low+1;
        int len2=high-mid;
        vector<long long> temp1(len1);
        vector<long long> temp2(len2);
        for(int i=0;i<len1;i++){
            temp1[i]=nums[low+i];
        }
        for(int i=0;i<len2;i++){
            temp2[i]=nums[i+mid+1];
        }
        int count=countPairs(temp1,len1,temp2,len2);
        int l1=0,l2=0;
        int ind=low;
        while(l1<len1 && l2<len2){
            if(temp1[l1]<=temp2[l2]){
                nums[ind++]=temp1[l1++];
            }
            else{
                nums[ind++]=temp2[l2++];
            }
        }
        while(l1<len1){
            nums[ind++]=temp1[l1++];
        }
        while(l2<len2){
            nums[ind++]=temp2[l2++];
        }
        return count;
    }
    int mergeSort(vector<int>& nums,int low,int high){
        int ans=0;
        if(low<high){
            int mid=(low+high)/2;

            ans+=mergeSort(nums,low,mid);
            ans+=mergeSort(nums,mid+1,high);

            ans+=merge(nums,low,mid,high);
        }
        return ans;
    }
public:
    int reversePairs(vector<int>& nums) {
        int n=nums.size();
        return mergeSort(nums,0,n-1);
    }
};
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