Solutions

1. Brute Force

class Solution:
    def largestGoodInteger(self, num: str) -> str:
        res = ""
        val = 0

        for i in range(len(num) - 2):
            if num[i] == num[i + 1] == num[i + 2]:
                tmp = num[i : i + 3]
                if val <= int(tmp):
                    val = int(tmp)
                    res = tmp

        return res

Time & Space Complexity

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

2. Iteration

class Solution:
    def largestGoodInteger(self, num: str) -> str:
        res = "0"

        for i in range(len(num) - 2):
            if num[i] == num[i + 1] == num[i + 2]:
                res = max(res, num[i : i + 3])

        return "" if res == "0" else res

Time & Space Complexity

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

3. Iteration (Optimal)

class Solution:
    def largestGoodInteger(self, num: str) -> str:
        res = -1

        for i in range(len(num) - 2):
            if num[i] == num[i + 1] == num[i + 2]:
                res = max(res, int(num[i]))
        return str(res) * 3 if res != -1 else ""

Time & Space Complexity

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