Is Sequential Digits easy or hard?

Sequential Digits is rated Medium on LeetCode but is often considered an easier medium problem. The main challenge is recognizing the digit pattern and generating numbers efficiently instead of brute-forcing the entire range.

How to solve Sequential Digits in O(1)?

Generate candidate numbers using a sliding window over the string "123456789" or build them incrementally starting from digits 1–9. Each generated number is checked against the range [low, high]. Because the total candidates are bounded, the algorithm effectively runs in constant time.

What is the best approach for Sequential Digits?

The sliding window or BFS generation approach is usually considered the best. Instead of checking every number in the range, you directly generate valid sequential numbers such as 123 or 4567. Since there are fewer than 40 possible sequential numbers overall, the algorithm runs in constant time and is very efficient.

Is Sequential Digits asked at Google/Amazon/Meta?

Sequential Digits is a common enumeration-style problem that has appeared in interview prep lists and coding assessments. Variations of pattern-based number generation and digit manipulation are frequently asked by companies like Amazon and Google in coding interviews.

What data structure is used in Sequential Digits?

Most solutions rely on simple arrays or lists to store generated numbers. Some implementations use a queue to perform breadth-first generation of sequential numbers. The problem focuses more on enumeration logic than complex data structures.

What is the time complexity of Sequential Digits?

The time complexity is O(1). The number of possible sequential digit values is fixed because digits range only from 1 to 9. Even if you generate all possibilities, the total count is small and independent of the input size.

Sequential Digits Python or Java solution approach?

In Python or Java, the most common solution uses a sliding window over the string "123456789" to generate substrings and convert them to integers. Another approach uses a queue to build numbers like 1 → 12 → 123. Both implementations run in constant time with minimal memory.

Description Hints 2 Solution Video 15:02

1291. Sequential Digits

Medium68.6% acceptance3 companies

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 1:

Input: low = 100, high = 300
Output: [123,234]

Example 2:

Input: low = 1000, high = 13000
Output: [1234,2345,3456,4567,5678,6789,12345]

Constraints:

10 <= low <= high <= 10^9

Asked at 3 companies

F5 Networks

Google Meta

Topics

Enumeration

Hints open one at a time

Hint 1

Generate all numbers with sequential digits and check if they are in the given range.

Hint 2

Fix the starting digit then do a recursion that tries to append all valid digits.

15:02

Still stuck?Read 5 approaches, or watch a walkthrough.

Open solution

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1class Solution:
2    def sequentialDigits(self, low: int, high: int) -> List[int]:
3        

Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

100
300

Input

1000
13000

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