K-diff Pairs in an Array LeetCode solution in C++

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class Solution {
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 public:
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  int findPairs(vector<int> &nums, int k) {
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    unordered_map<int, int> a;
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    for (int i : nums) a[i]++;
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    int ans = 0;
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    for (auto x : a) {
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      if (k == 0) {
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        if (x.second > 1) ans++;
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      } else if (a.find(x.first + k) != a.end())
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        ans++;
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    }
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    return ans;
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  }
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};