<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 Solution {
public:
    void update(int idx, vector<int>& bitTree, int n) {
        while(idx <= n) {
            bitTree[idx]++;
            idx += (idx&-idx);
        }
    }

    int query(int idx, vector<int>& bitTree) {
        int ans = 0;
        while(idx > 0) {
            ans += bitTree[idx];
            idx -= (idx&-idx);
        }
        return ans;
    }

    int reversePairs(vector<int>& nums) {
        int n = nums.size();
        map<long, int> hash;
        int count = 1;
        vector<int> numsCopy = nums;
        sort(numsCopy.begin(), numsCopy.end());
        for(int it : numsCopy) {
            if(hash[it] == 0) {
                hash[it] = count++;
            }
        }
        vector<int> bitTree(n+1, 0);
        int ans = 0;
        for(auto val : nums) {
            auto itr = hash.upper_bound(2L*val);
            if(itr != hash.end()) {
                int hashedVal = (*itr).second;
                ans += query(n, bitTree) - query(hashedVal-1, bitTree);
            }
            update(hash[val], bitTree, n);
        }
        return ans;
    }
};
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