Find Lucky Integer in an Array LeetCode solution in C++

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class Solution {
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 public:
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  int findLucky(vector<int> &arr) {
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    // sort(arr.begin(),arr.end());
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    map<int, int> mp;
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    vector<int> v;
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    for (int i = 0; i < arr.size(); i++) {
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      mp[arr[i]]++;
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    }
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    for (auto x : mp) {
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      if (x.first == x.second) {
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        v.push_back(x.first);
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      }
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    }
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    int mx = -1;
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    for (int i = 0; i < v.size(); i++) {
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      mx = max(mx, v[i]);
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    }
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    return mx;
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  }
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};