<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 {
    long long merge(vector<int>& arr,int start1,int start2,int end){
    int i=start1,j=start2;
    int ans=0;
    while(i<start2 && j<=end){
        while(i<start2 && arr[i]<=(1LL*2*arr[j]))   i++;
        while(j<=end && arr[i]>(1LL*2*arr[j])){
            ans+=(start2-i);
            j++;
        }
    }
    i=0,j=start2-start1;
    vector<int>sorted;
    for(int k=start1;k<=end;k++){
        sorted.push_back(arr[k]);
    }
    int k=j,len=end-start1+1;
    while(i<k && j<len){
        if(sorted[i]<=sorted[j])    arr[start1++]=sorted[i++];
        else    arr[start1++]=sorted[j++];
    }
    while(i<k)  arr[start1++]=sorted[i++];
    while(j<len)    arr[start1++]=sorted[j++];
    return ans;
}
long long mergeSort(vector<int>& arr,int low,int high){
    if(low==high)   return 0;
    int mid=low+((high-low)/2);
    long long ans1 = mergeSort(arr,low,mid);
    long long ans2 = mergeSort(arr,mid+1,high);
    return ans1+ans2+merge(arr,low,mid+1,high);
}
public:
    int reversePairs(vector<int>& nums) {
        int n=nums.size();
        return mergeSort(nums,0,n-1);
    }
};
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