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:

• 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".


Example 2:

Input: num = "2300019"
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:

Time complexity: O(n)
Space complexity: O(1)

## C++

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