<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:
    struct TrieNode {
        vector<TrieNode*> bits;
        int count;
        TrieNode() {
            count = 0;
            bits.assign(2, nullptr);
        }
    };

    struct Trie {
        TrieNode *head;
        Trie() {
            head = new TrieNode();
        }

        void insert(long long num) {
            TrieNode* dummy = head;
            for(int i = 32; i >= 0; i--) {
                int set = (num >> i) & 1ll;
                if(dummy->bits[set] == nullptr) {
                    dummy->bits[set] = new TrieNode();
                } 
                dummy = dummy->bits[set];
                dummy->count++;
            }
        }

        int countMoreThan(long long num) {
            TrieNode* dummy = head;
            int count = 0;
            for(int i = 32; i >= 0; i--) {
                int set = (num >> i) & 1ll;
                if(!set && dummy->bits[1]) {
                    count += dummy->bits[1]->count;
                }
                if(dummy->bits[set]) {
                    dummy = dummy->bits[set];
                } else {
                    break;
                }
            }
            return count;
        }
    };

    int reversePairs(vector<int>& A) {
        int n = A.size(), res = 0;
        Trie* t = new Trie();
        long long offset = 2ll*INT_MAX+2ll; // Adjust according to your input range

        for(int i = 0; i < n; i++) {
            res += t->countMoreThan(2ll * A[i]+ offset);
            t->insert(A[i] + offset);
        }
        return res;
    }
};

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