Leetcode #248: Strobogrammatic Number III
In this guide, we solve Leetcode #248 Strobogrammatic Number III 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
Given two strings low and high that represent two integers low and high where low <= high, return the number of strobogrammatic numbers in the range [low, high]. A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).
Quick Facts
- Difficulty: Hard
- Premium: Yes
- Tags: Recursion, Array, String
Intuition
We need to scan characters while tracking positions or counts.
A simple state machine keeps the logic precise.
Approach
Iterate through the string once and update the state for each character.
Use a map or array if you need fast lookups.
Steps:
- Iterate through characters.
- Maintain necessary state.
- Build or validate the output.
Example
Input: low = "50", high = "100"
Output: 3
Python Solution
class Solution:
def strobogrammaticInRange(self, low: str, high: str) -> int:
def dfs(u):
if u == 0:
return ['']
if u == 1:
return ['0', '1', '8']
ans = []
for v in dfs(u - 2):
for l, r in ('11', '88', '69', '96'):
ans.append(l + v + r)
if u != n:
ans.append('0' + v + '0')
return ans
a, b = len(low), len(high)
low, high = int(low), int(high)
ans = 0
for n in range(a, b + 1):
for s in dfs(n):
if low <= int(s) <= high:
ans += 1
return ans
Complexity
The time complexity is . 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.