<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>


#define ll long long
class Solution {
public:

ll merge(ll l,ll mid,ll r,vector<ll>&v) {
    
       ll ans = 0;
        
        ll n = r-l+1;

        vector<ll>arr(n);
        
        ll cnt = 0;
        
        ll y = mid+1;
        ll x = l;

        ll b = y;
        
    
        while(x<=mid&&y<=r) {
            
            if(v[x]<=2*v[y]) {

                x++;   
            }
            
            else {
              
                ans+=(mid-x+1);

                 y++;
            }
            
    
        }

        cnt = 0;
        x = l;
        y = mid+1;

          while(x<=mid&&y<=r) {
            
            if(v[x]<v[y]) {
                arr[cnt] = v[x];
                x++;

            }
            
            else {
               arr[cnt] = v[y];
                 y++;
            }
            
            cnt++;
        }
        
        while(x<=mid) {
             
             arr[cnt] = v[x];
                x++;
                cnt++;
        }
        
        while(y<=r) {
              
               arr[cnt] = v[y];
                y++;
                cnt++;
        }
        
        for(ll i = l;i<=r;i++) {
            v[i] = arr[i-l];
        }
        
        return ans;
}

ll mergesort(ll l,ll r,vector<ll>&v) {
    
    ll ans = 0;
        
    
    if(l<r) {
        
        
    ll  mid = (l+r)/2;
    
   ans+= mergesort(l,mid,v);
    
   ans+= mergesort(mid+1,r,v);
    
   ans+= merge(l,mid,r,v);
    }
    
    return ans;
}
    int reversePairs(vector<int>& nums) {
        

        vector<ll>arr(nums.begin(),nums.end());

        return mergesort(0,nums.size()-1,arr);
    }
};
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