Find All Duplicates in an Array LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> findDuplicates(vector<int> &nums) {
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    int n = nums.size();
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    vector<int> a;
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    vector<int> arr(n + 1, 0);
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    for (int i = 0; i < nums.size(); i++) arr[nums[i]]++;
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    for (int j = 0; j <= n; j++)
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      if (arr[j] == 2) a.push_back(j);
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    return a;
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  }
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};