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class Solution
{
public:
    int missingNumber(vector<int> &arr)
    {
        int n = arr.size();
        //         int diff = arr[1] - arr[0];
        //         int cnt = 0;
        //         int ans;
        //         for(int i = 2; i < n; i++) {
        //             int tmp = arr[i] - arr[i - 1];
        //             if(diff != tmp) {
        //                 cnt++;
        //                 ans = arr[i - 1] + diff;
        //             }
        //         }

        //         if(cnt != 1) {
        //             cnt = 0;
        //             int diff = arr[2] - arr[1];
        //             ans = arr[0] + diff;
        //         }

        //         if(n == 3) {
        //             int diff = arr[1] - arr[0];
        //             int diff2 = arr[2] - arr[1];

        //             if(abs(diff) < abs(diff2))
        //                 ans = arr[1] + diff;
        //             else
        //                 ans = arr[0] + diff2;
        //         }

        //         return ans;

        for (int i = 2; i < n; i++) {
            if (arr[i] - arr[i - 1] != arr[i - 1] - arr[i - 2]) {
                return arr[i] + arr[i - 2] - arr[i - 1];
            }
        }

        return 0; // 0, 0, 0, 0...
    }
};