Largest 3-Same-Digit Number in String

Largest 3-Same-Digit Number in String - Problem

String Easy

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.

Input & Output

Example 1 — Multiple Good Integers

$ Input: num = "6777133"

› Output: "777"

💡 Note: The good integers are "777" and "333". Since "777" > "333", we return "777".

Example 2 — No Good Integer

$ Input: num = "2300019"

› Output: "000"

💡 Note: The only good integer is "000" (three consecutive zeros).

Example 3 — No Valid Triplets

$ Input: num = "42352338"

› Output: ""

💡 Note: No substring of length 3 consists of only one unique digit.

Constraints

3 ≤ num.length ≤ 1000
num only consists of digits.

Visualization

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Asked in

G Google 15 a Amazon 12 M Microsoft 8

The key insight is to scan through the string and identify 3 consecutive identical digits, then keep track of the lexicographically largest such substring. Best approach is the optimized single pass with direct character comparison. Time: O(n), Space: O(1).

Common Approaches

✓ Brute Force - Check All Substrings

⏱️ Time: O(n) Space: O(1)

Iterate through all possible starting positions and check if the 3-character substring has all identical digits. Keep track of the maximum valid substring found.

Optimized Single Pass

⏱️ Time: O(n) Space: O(1)

Instead of checking every substring, we scan the string and when we encounter three consecutive identical characters, we immediately know we have a valid candidate. We compare it with our current best and continue.

Brute Force - Check All Substrings — Algorithm Steps

Step 1: Loop through all valid starting positions (0 to len-3)
Step 2: For each position, check if next 3 characters are identical
Step 3: If valid, compare with current maximum and update if larger

Visualization

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Step-by-Step Walkthrough

Scan Windows

Check each 3-character window from left to right

Validate

Check if all 3 characters are identical

Track Maximum

Keep the lexicographically largest valid substring

Code -

solution.c — C

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

char* solution(char* num) {
    int len = strlen(num);
    char* maxGood = malloc(4 * sizeof(char));
    strcpy(maxGood, "");
    
    for (int i = 0; i <= len - 3; i++) {
        if (num[i] == num[i+1] && num[i+1] == num[i+2]) {
            char substring[4];
            strncpy(substring, num + i, 3);
            substring[3] = '\0';
            
            if (strcmp(substring, maxGood) > 0) {
                strcpy(maxGood, substring);
            }
        }
    }
    
    return maxGood;
}

int main() {
    char num[1001];
    fgets(num, sizeof(num), stdin);
    num[strcspn(num, "\n")] = 0;
    
    char* result = solution(num);
    printf("%s\n", result);
    free(result);
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single loop through string positions, constant work per position

✓ Linear Growth

Space Complexity

O(1)

Only using a few variables to store the result

✓ Linear Space

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Smart Actions

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Input & Output

Constraints

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Related Problems

Common Approaches

Brute Force - Check All Substrings — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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