Return the Greatest Repeated-Digit Triple in a Long Number
Company: Google
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: easy
Interview Round: Other
### Problem
Find contiguous length-three substrings of `num` containing only one repeated digit. Return the greatest such triple as a three-character string, or the empty string when none exists. Leading zeroes remain meaningful in the returned spelling.
```cpp
class Solution {
public:
string largestGoodInteger(string num);
};
```
### Examples
**Example 1:**
```text
Input: num = "2227771"
Output: "777"
Explanation: The triples "222" and "777" qualify, and "777" is greater.
```
**Example 2:**
```text
Input: num = "10002"
Output: "000"
Explanation: The triple "000" is valid and must retain all three zero characters.
```
### Constraints
- `3 <= num.length <= 1000`.
- num consists of decimal digits.
### Clarifying Questions
- Does a triple have to be contiguous? Yes; three equal digits at separate positions do not qualify.
- Is `"000"` different from no result? Yes. Return all three zero characters when it is the greatest qualifying triple.
- Can triples overlap? Yes; a run such as `"4444"` contains two occurrences of the same candidate.
```hint Check the boundary
Check the difference between a three-character result and an empty result.
```
Overview: Find the greatest three-character substring made of one repeated digit. Preserve leading zeroes and return an empty string when no triple qualifies.