Count Array Pairs Divisible by K LeetCode solution in C++

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typedef long long ll;
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class Solution {
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 public:
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  long long countPairs(vector<int> &nums, int k) {
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    unordered_map<ll, ll> gcdCount;
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    ll ans = 0;
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    for (ll i = 0; i < nums.size(); ++i) {
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      ll currgcd = __gcd(nums[i], k);
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      for (auto &[gc_d, count] : gcdCount)
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        if ((currgcd * gc_d) % k == 0) ans += count;
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      gcdCount[currgcd]++;
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    }
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    return ans;
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  }
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};