<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 {
    int cnt;
    void merge(int l, int r, int mid, vector<long long> &nums){
        int sz_l = mid-l+1;
        int sz_r = r-mid;
        vector<long long> temp_l(sz_l+1);
        temp_l[sz_l]=2*(long long)INT_MAX+10ll;
        vector<long long> temp_r(sz_r+1);
        temp_r[sz_r]=2*(long long)INT_MAX+10ll;
        for(int i=l; i<=mid; i++) temp_l[i-l]=nums[i];
        for(int i=mid+1; i<=r; i++) temp_r[i-(mid+1)]=nums[i];
        int cur=l;
        int left=0;
        int right=0;
        while(left!=sz_l || right!=sz_r){
            if(temp_l[left]<=temp_r[right]){
                nums[cur]=temp_l[left];
                cur++;
                left++;
            }
            else{
                nums[cur]=temp_r[right];
                cur++;
                right++;
            }
        }
        left=0;
        right=0;
        while(left!=sz_l || right!=sz_r){
            if(temp_l[left]<=2ll*temp_r[right]){
                left++;
            }
            else{
                right++;
                cnt += sz_l - left;
            }
        }
    }
    void mergeSort(int l, int r, vector<long long> &nums){
        if(l==r) return;
        int mid = (l+r)/2;
        mergeSort(l,mid,nums);
        mergeSort(mid+1,r,nums);
        merge(l,r,mid,nums);
    }
public:
    int reversePairs(vector<int>& nums) {
        cnt=0;
        vector<long long> neww;
        for(int e: nums){
            neww.push_back((long long)e);
            // cout<<e<<" ";
        }
        // cout<<"\n";
        int n=nums.size();
        mergeSort(0,n-1,neww);
        return cnt;
    }
};
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