<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>

struct Node{
    long long int count;
    long long int l;
    long long int r;
    Node* left;
    Node* right;
    Node(long long int l,long long int r,long long int count): count(count), l(l) ,r(r),left(nullptr),right(nullptr){}
};
Node* root;

void insert(Node* node,long long int val){
    // cout << node->l << " " << node->r <<endl;
    if(val<node->l||val>=node->r)return;
    node->count++;
    if(node->l+1==node->r)return;
    if(node->left==nullptr)node->left = new Node(node->l,(node->l+node->r)/2,0);
    if(node->right==nullptr)node->right = new Node((node->l+node->r)/2,node->r,0);
    insert(node->left,val);
    insert(node->right,val);
}
long long int query(Node* node,long long int x){
    if(node==nullptr)return 0;
    if(x>=node->r) return 0;
    if(x<node->l)return node->count;
    long long int ans = 0;
    ans+=query(node->left,x);
    ans+=query(node->right,x);
    return ans;
}

class Solution {
public:
    int reversePairs(vector<int>& nums) {
        root =  new Node(INT_MIN,(long long int)INT_MAX+1,0);
        long long int ans = 0;
        insert(root,nums[0]);
        for(int i=1;i<nums.size();i++){
            ans+= query(root,2* (long long int )nums[i]);
            cout << ans <<endl;
            insert(root,nums[i]);
        }
        // cout << root->count << endl;
        return ans;
    }
};
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