2264. Largest 3-Same-Digit Number in String

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.

class Solution {
public:
    string largestGoodInteger(string num) {
        string res = "";
        for(int i = 0; i < num.length() - 2; i++) {
            if(num[i] == num[i+1] and num[i+1] == num[i+2]) {
                res = max(res, string(3,num[i]));
            }
        }
        return res;
    }
};