Find All Numbers Disappeared in an Array LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> findDisappearedNumbers(vector<int> &nums) {
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    vector<int> res;
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    int n = nums.size();
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    for (int i = 0; i < n; i++) {
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      if (nums[abs(nums[i]) - 1] > 0) {
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        nums[abs(nums[i]) - 1] *= -1;
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      }
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    }
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    for (int i = 0; i < n; i++) {
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      if (nums[i] > 0) res.push_back(i + 1);
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    }
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    return res;
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  }
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};