Sequential Digits - Practice Coding Problems

Sequential Digits - Problem

Enumeration Medium

An integer has sequential digits if and only if each digit in the number is one more than the previous digit.

Return a sorted list of all the integers in the range [low, high] inclusive that have sequential digits.

Example: The number 234 has sequential digits because 2→3→4 where each digit is exactly one more than the previous. The number 235 does NOT have sequential digits because 3→5 skips 4.

Input & Output

Example 1 — Basic Range

$ Input: low = 100, high = 300

› Output: [123,234]

💡 Note: 123 has sequential digits (1→2→3), 234 has sequential digits (2→3→4). No other numbers in range [100,300] have sequential digits.

Example 2 — Wider Range

$ Input: low = 1000, high = 13000

› Output: [1234,2345,3456,4567,5678,6789,12345]

💡 Note: All 4-digit sequential numbers (1234-6789) and the 5-digit number 12345 fall within this range.

Example 3 — Single Digits

$ Input: low = 1, high = 10

› Output: [1,2,3,4,5,6,7,8,9]

💡 Note: All single digits from 1-9 have sequential digits by definition (only one digit).

Constraints

10 ≤ low ≤ high ≤ 10⁹

Visualization

Tap to expand

Understanding the Visualization

Input Range

Given low=100, high=300

Check Pattern

Find numbers where digits go n, n+1, n+2, ...

Output

Return [123, 234] - both have sequential digits

Key Takeaway

🎯 Key Insight: Only 36 sequential digit numbers exist total - generate them all instead of checking every number in range

Asked in

G Google 15 a Amazon 12 M Microsoft 8

The key insight is that there are only 36 possible sequential digit numbers total (1-9, 12-89, 123-789, etc.). Instead of checking every number in the range, we can generate all sequential numbers using the base pattern '123456789' and filter by the given range. Best approach generates all possibilities in O(1) time since the count is fixed, then filters by range.

Common Approaches

Approach	Time	Space	Notes
✓ Generate Sequential Numbers	O(1)	O(k)	Generate all possible sequential digit numbers, then filter by range
Brute Force Check	O(n × d)	O(k)	Check every number in the range to see if it has sequential digits

Generate Sequential Numbers — Algorithm Steps

Step 1: Use base string '123456789' as template
Step 2: Generate all possible substrings of length 1-9
Step 3: Filter generated numbers that fall within [low, high] range

Visualization

Tap to expand

Step-by-Step Walkthrough

Base Pattern

Use '123456789' as template

Extract Substrings

Get all possible consecutive digit sequences

Filter Range

Keep only numbers within [low, high]

Code -

solution.c — C

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

int compare(const void *a, const void *b) {
    return (*(int*)a - *(int*)b);
}

int main() {
    int low, high;
    scanf("%d", &low);
    scanf("%d", &high);
    
    int result[100];
    int count = 0;
    char base[] = "123456789";
    
    // Generate all possible sequential digit numbers
    for (int length = 1; length <= 9; length++) {
        for (int start = 0; start <= 9 - length; start++) {
            char substr[10];
            strncpy(substr, base + start, length);
            substr[length] = '\0';
            int num = atoi(substr);
            if (num >= low && num <= high) {
                result[count++] = num;
            }
        }
    }
    
    qsort(result, count, sizeof(int), compare);
    
    printf("[");
    for (int i = 0; i < count; i++) {
        printf("%d", result[i]);
        if (i < count - 1) printf(",");
    }
    printf("]\n");
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(1)

Fixed 36 possible sequential numbers to generate regardless of range

✓ Linear Growth

Space Complexity

O(k)

k sequential numbers found in the result list

✓ Linear Space

25.0K Views

Medium Frequency

~15 min Avg. Time

892 Likes

Ln 1, Col 1

Smart Actions

💡 Explanation

AI Ready

💡 Suggestion Tab to accept Esc to dismiss

// Output will appear here after running code

Code Editor Closed

Click the red button to reopen

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Generate Sequential Numbers — Algorithm Steps

Visualization

Code -

Time & Space Complexity

Select Compiler