<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>

int SIZE = 1;
template <typename T>
T getBetween(vector<T>& v, int l, int r){
    if(r<l) return 0;
    if(l==r) return v[l+SIZE];
    l += SIZE;
    r += SIZE;
    T between = v[l]+v[r];
    while(l<r-1){
        if(l%2==0) between += v[l+1];
        if(r%2!=0) between += v[r-1];
        l>>=1;
        r>>=1;
    }
    return between;
}

template <typename T>
void update(vector<T>& v, int pos, T value){
    pos += SIZE;
    v[pos] += value;
    pos/=2;
    while(pos){
        v[pos] += value;
        pos >>= 1;
    }
}

class Solution {
public:
    int reversePairs(vector<int>& nums) {
        SIZE = 1;
        map<long long,int> indexer;
        for(int num:nums) indexer[num]=0;
        int nowIndex = 0;
        for(auto& x:indexer){
            x.second=nowIndex++;
        }
        while(SIZE<=nowIndex) SIZE<<=1;
        vector<int> cntTree(SIZE*2);
        long long ans = 0;
        for(long long num:nums){
            int idx = 0;
            auto x = indexer.upper_bound(2*num);
            if(x==indexer.end()) idx = nowIndex;
            else idx = x->second;
            ans += getBetween(cntTree, idx, nowIndex);
            update(cntTree, indexer[num], 1);
        }
        return ans;
    }
};

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