Count Special Quadruplets LeetCode solution in C++

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class Solution {
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 public:
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  int countQuadruplets(vector<int> &nums) {
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    int res = 0;
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    int len = nums.size();
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    unordered_map<int, int> count;
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    count[nums[len - 1] - nums[len - 2]] = 1;
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    for (int b = len - 3; b >= 1; b--) {
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      for (int a = b - 1; a >= 0; a--) {
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        res += count[nums[a] + nums[b]];
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      }
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      for (int x = len - 1; x > b; x--) {
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        count[nums[x] - nums[b]]++;
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      }
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    }
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    return res;
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  }
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};