2264. Largest 3-Same-Digit Number in String 

Description

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 = "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.

Solutions

Solution 1: Enumeration

We can enumerate each digit $i$ from large to small, where $0 \le i \le 9$, and then check whether the string $s$ consisting of three consecutive $i$ is a substring of $num$. If it is, we directly return $s$.

If we have enumerated all the possible values of $i$ and still haven't found a substring that satisfies the condition, we return an empty string.

The time complexity is $O(10 \times n)$, where $n$ is the length of the string $num$. The space complexity is $O(1)$.

Python3

class Solution:
    def largestGoodInteger(self, num: str) -> str:
        for i in range(9, -1, -1):
            if (s := str(i) * 3) in num:
                return s
        return ""

Java

class Solution {
    public String largestGoodInteger(String num) {
        for (int i = 9; i >= 0; i--) {
            String s = String.valueOf(i).repeat(3);
            if (num.contains(s)) {
                return s;
            }
        }
        return "";
    }
}

C++

class Solution {
public:
    string largestGoodInteger(string num) {
        for (char i = '9'; i >= '0'; --i) {
            string s(3, i);
            if (num.find(s) != string::npos) {
                return s;
            }
        }
        return "";
    }
};

Go

func largestGoodInteger(num string) string {
	for c := '9'; c >= '0'; c-- {
		if s := strings.Repeat(string(c), 3); strings.Contains(num, s) {
			return s
		}
	}
	return ""
}

TypeScript

function largestGoodInteger(num: string): string {
    for (let i = 9; i >= 0; i--) {
        const s = String(i).repeat(3);
        if (num.includes(s)) {
            return s;
        }
    }
    return '';
}