Minimum Operations to Make a Uni-Value Grid LeetCode solution in C++

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class Solution {
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 public:
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  int minOperations(vector<vector<int>> &grid, int x) {
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    vector<int> nums;
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    int m = grid.size(), n = grid[0].size();
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    for (int i = 0; i < m; i++)
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      for (int j = 0; j < n; j++) nums.push_back(grid[i][j]);
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    sort(nums.begin(), nums.end());
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    int target = nums[m * n / 2], ans = 0;
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    for (int i = m * n - 1; i >= 0; i--) {
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      if (abs(nums[i] - target) % x != 0)
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        return -1;
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      else
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        ans += abs(nums[i] - target) / x;
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    }
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    return ans;
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  }
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};