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You are given a string num representing a large integer. An integer is good if it meets the following conditions:

It is a substring of num with length 3. It consists of only one unique digit. Return the maximum good integer as a string or an empty string "" if no such integer exists.

Note:

A substring is a contiguous sequence of characters within a string. There may be leading zeroes in num or a good integer.

Example 1:

Input: num = "6777133339" Output: "777" Explanation: There are two distinct good integers: "777" and "333". "777" is the largest, so we return "777".

 string largestGoodInteger(string num) {
        int result=-1;

        for(int i=0;i+2<num.length();i++)
            if(num[i]==num[i+1] && num[i]==num[i+2])
                result = max(result,static_cast<int>(num[i] - '0'));

        return (result==-1)? "": string(3,'0'+result);
    }