Leetcode #1291: Sequential Digits
In this guide, we solve Leetcode #1291 Sequential Digits in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
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.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Enumeration
Intuition
The constraints allow a direct scan that keeps only the essential state.
By translating the requirements into a clean loop, the logic stays easy to reason about.
Approach
Iterate through the data once, updating the state needed to compute the answer.
Return the final state after the traversal is complete.
Steps:
- Parse the input.
- Iterate and update state.
- Return the computed answer.
Example
Input: low = 100, high = 300
Output: [123,234]
Python Solution
class Solution:
def sequentialDigits(self, low: int, high: int) -> List[int]:
ans = []
for i in range(1, 9):
x = i
for j in range(i + 1, 10):
x = x * 10 + j
if low <= x <= high:
ans.append(x)
return sorted(ans)
Complexity
The time complexity is O(n). The space complexity is O(1) to O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.