LeetCode #2264 — EASY

Largest 3-Same-Digit Number in String

Build confidence with an intuition-first walkthrough focused on core interview patterns fundamentals.

Solve on LeetCode

The Problem

Problem Statement

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.

Step 01

Brute Force Baseline

Problem summary: 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.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: General problem-solving

Example 1

"6777133339"

Example 2

"2300019"

Example 3

"42352338"

Core Insight

What unlocks the optimal approach

We can sequentially check if “999”, “888”, “777”, … , “000” exists in num in that order. The first to be found is the maximum good integer.
If we cannot find any of the above integers, we return an empty string “”.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #2264: Largest 3-Same-Digit Number in String
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 "";
    }
}

// Accepted solution for LeetCode #2264: Largest 3-Same-Digit Number in String
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 ""
}

# Accepted solution for LeetCode #2264: Largest 3-Same-Digit Number in String
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 ""

// Accepted solution for LeetCode #2264: Largest 3-Same-Digit Number in String
/**
 * [2264] Largest 3-Same-Digit Number in String
 *
 * 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 = "6<u>777</u>133339"
 * Output: "777"
 * Explanation: There are two distinct good integers: "777" and "333".
 * "777" is the largest, so we return "777".
 *
 * Example 2:
 *
 * Input: num = "23<u>000</u>19"
 * 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.
 *
 */
pub struct Solution {}

// problem: https://leetcode.com/problems/largest-3-same-digit-number-in-string/
// discuss: https://leetcode.com/problems/largest-3-same-digit-number-in-string/discuss/?currentPage=1&orderBy=most_votes&query=

// submission codes start here

impl Solution {
    pub fn largest_good_integer(num: String) -> String {
        num.as_bytes()
            .windows(3)
            .filter(|bytes| bytes.iter().all(|&b| b == bytes[0])) //[[55,55,55]], [57,57,57]]
            .map(|b| b[0])
            .max()
            .map(|b| (b as char).to_string().repeat(3))
            .unwrap_or_default() //"999"
    }
}

// submission codes end

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_2264_example_1() {
        let num = "6777133339".to_string();

        let result = "777".to_string();

        assert_eq!(Solution::largest_good_integer(num), result);
    }

    #[test]
    fn test_2264_example_2() {
        let num = "2300019".to_string();

        let result = "000".to_string();

        assert_eq!(Solution::largest_good_integer(num), result);
    }

    #[test]
    fn test_2264_example_3() {
        let num = "42352338".to_string();

        let result = "".to_string();

        assert_eq!(Solution::largest_good_integer(num), result);
    }
}

// Accepted solution for LeetCode #2264: Largest 3-Same-Digit Number in String
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 '';
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n)

Space

O(1)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED

O(n) time

O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.