🧠 Solving LeetCode Until I Become Top 1% — Day `59`

🔹 Problem: Largest 3-Same-Digit Number in String

Difficulty: Easy
Tags: #String #Greedy

📝 Problem Summary

We are given a numeric string num.
We need to find the largest substring of length 3 where all digits are the same (like "777" or "333").
If there’s no such substring, return an empty string "".

🧠 My Thought Process

Brute Force Idea:
Check every substring of length 3, verify if all three characters are equal, and keep track of the largest. This would work but requires many slice operations.
Optimized Strategy:
- Traverse the string once while counting consecutive identical digits.
- When we find exactly three consecutive same digits, compare with the current maximum digit found.
- Store only the best digit (no need to store the whole substring).
- Build the final answer at the end by repeating that digit 3 times.
Algorithm Used:

Single-pass scan + max tracking.

⚙️ Code Implementation (Python)

class Solution:
    def largestGoodInteger(self, num: str) -> str:
        count = 0
        prev = 'X'   # sentinel value
        maxd = ' '   # store max digit found

        for c in num:
            if c == prev:
                count += 1
            else:
                count = 1
            if count == 3:
                maxd = max(c, maxd)
            prev = c

        return "" if maxd == ' ' else maxd * 3

⏱️ Time & Space Complexity

Time: O(n) — single scan over num.
Space: O(1) — constant variables.

🧩 Key Takeaways

✅ Consecutive character counting is often more efficient than substring slicing.
💡 Using max() on characters works directly since '9' > '8' > ... > '0' in ASCII.
💭 This approach can be adapted for problems like finding the longest consecutive character sequence.