<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 {
    class FenwickTree {
        public:
            FenwickTree(int size) {
                _size = size;
                _vals = vector<int>(size + 1, 0);
            }

            void update(int x, int val) {
                // int t = 0;
                for(; x <= _size; x += (x & (-x))) {
                    // cout << x <<' '<< _size <<' '<< t <<endl;
                    _vals[x] += val;
                    // t++;
                    // if (t == 1000) return;
                }
            }

            int get(int x) {
                if (x == 0) return 0;
                int ans = 0;
                for (; x; x -= (x & (-x)))
                    ans += _vals[x];
                return ans;
            }

        private:
            int _size;
            vector<int> _vals;
    };

public:
    int reversePairs(vector<int>& nums) {
        vector<long long> com;
        for (auto const& num: nums) {
            com.push_back(num);
            com.push_back(2ll*num);
        }

        sort(com.begin(), com.end());

        int n = com.size();
        FenwickTree *tree = new FenwickTree(n);
        int ret = 0;

        for (auto const& num: nums) {
            int x = lower_bound(com.begin(), com.end(), 2ll*num) - com.begin();
            x = n - x;

            int tmp = tree->get(x-1);
            ret += tmp;
            // cout << num <<' '<< x <<' '<< tmp <<endl;
            x = lower_bound(com.begin(), com.end(), num) - com.begin();
            x = n-x;
            tree->update(x, 1);
        }

        return ret;
    }
};
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