示例：

输入：arr = [1,2,2,6,6,6,6,7,10]
输出：6


提示：

1 <= arr.length <= 10^4
0 <= arr[i] <= 10^5

class Solution {
public:
    int findSpecialInteger(vector<int>& arr) {
        int n = arr.size(); int len = n/4;
        for(int i = 0; i < n;i++){
            if(i+len < n && arr[i] == arr[i+len]){
                return arr[i];
            }
        }

        return -1;
    }
};