<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 Node {
    public:
    Node* l;
    Node* r;
    long long val;
    long long left;
    long long right;
   Node(long long left, long long right) {
    this->left = left;
    this->right = right;
    this->val = 0;
    this->l = NULL;
    this->r = NULL;
}
};

class Solution {
public:

    int query(Node* head, long long index) {
        if(head==NULL) 
        return 0;
        if(head->left > index) {
            return 0;
        }
        if(head->right <= index) {
            return head->val;
        }
        return query(head->l, index)+query(head->r, index);
    }

    void update(Node* head, long long index) {
        if(index > head->right || index < head->left) {
            return;
        }
        head->val++;
        if(head->left == head->right) return;
        long long mid=head->left+head->right;
        mid/=2;
        if(head->left+1 ==  head->right) 
        mid=head->left;
        if(!head->l) 
        head->l=new Node(head->left, mid);
        if(!head->r)
        head->r=new Node(mid+1, head->right);
        update(head->l, index);
        update(head->r, index);
    }
    int reversePairs(vector<int>& nums) {
        int n=nums.size();
        long long ans=0;
        Node* head= new Node(1ll*2*INT_MIN, 1ll*2*INT_MAX);
        for(int i=n-1;i>=0;i--) {
            ans+=query(head, 1ll*nums[i]-1);
            if(i!=0)
            update(head, 1ll*2*nums[i]);
        }
        return ans;
    }
};
200378