<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 CoordinateCompression{
    map<long long,int>mp;
    int cnt;
public:
    void add(long long x){mp[x];}
    void build(){
        cnt = 0;
        for(auto &p:mp){
            p.second = cnt++;
        }
    }
    int getIndex(long long x){
        if(mp.find(x)!=mp.end())return mp[x];
        return -1;
    }
    int size(){
        return cnt;
    }
    int getLo(long long x){
        auto it = mp.lower_bound(x);
        if(it==mp.begin())return -1;
        it--;
        return it->second;
    }
};
class FenwickTree{
    vector<int>bit;
    int n;
public:
    FenwickTree(int n){
        this->n =n;
        bit.assign(n,0);
    }
    void add(int i){
        for(int j=i;j<n;j=(j|(j+1))){
            bit[j]++;
        }
    }
    int sum(int r){
        int ans = 0;
        for(int j=r;j>=0;j = (j&(j+1))-1){
            ans+=bit[j];
        }
        return ans;
    }
};
class Solution {
public:
#define int long long
    int reversePairs(vector<int>& nums) {
        int n= nums.size();
        CoordinateCompression cc;
        for(int i=0;i<n;i++){
            cc.add(2*nums[i]);
        }
        cc.build();
        FenwickTree ft(cc.size());
        int cnt = 0;
        for(int i=n-1;i>=0;i--){
            int lo = cc.getLo(nums[i]);
            // cout<<lo<<endl;
            if(lo!=-1)cnt+=ft.sum(lo);
            ft.add(cc.getIndex(2*nums[i]));
        }
        return cnt;
    }
#undef int
    int reversePairs(vector<int>&nums){
        vector<long long>p;
        for(auto x:nums){
            p.push_back(x);
        }
        return reversePairs(p);
    }
};
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