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.

Example 1:

Input: num = "6*777133339"
**Output:* "777"
Explanation: There are two distinct good integers: "777" and "333".
"777" is the largest, so we return "777".

Example 2:

Input: num = "23*00019"
**Output:* "000"
Explanation: "000" is the only good integer.

Example 3:

Input: num = "42352338"
Output: ""
Explanation: No substring of length 3 consists of only one unique digit. Therefore, there are no good integers.

Constraints:

• 3 <= num.length <= 1000
• num only consists of digits.

SOLUTION:

class Solution:
    def largestGoodInteger(self, num: str) -> str:
        n = len(num)
        i = 0
        mnum = float('-inf')
        while i < n:
            ctr = 1
            while i < n - 1 and num[i] == num[i + 1]:
                i += 1
                ctr += 1
            i += 1
            if ctr >= 3:
                mnum = max(mnum, int(num[i - 1]))
        if mnum == float('-inf'):
            return ""
        return str(mnum) * 3