Building on the concept of strobogrammatic numbers, Strobogrammatic Number II asks you to find all strobogrammatic numbers of a specific length n. A strobogrammatic number is one that looks the same when rotated 180 degrees. For example, if n=2, the valid numbers are "11", "69", "88", and "96". Note that "00" is not a valid 2-digit number because it has leading zeros. This problem requires generating all possible combinations that satisfy the strobogrammatic property for a given length.
This question is a favorite at Uber and Google because it tests recursion and backtracking skills. Unlike the first version of the problem which was a simple validation, this version requires construction. It assesses how a candidate handles recursive branching and how they manage edge cases like leading zeros. It's a great example of how a simple concept can be extended into a more complex combinatorial problem.
The primary pattern for this problem is Recursion or Backtracking. To generate a number of length n, you can start by generating all strobogrammatic numbers of length n-2 and then wrapping them with valid strobogrammatic pairs (00, 11, 88, 69, 96). The base cases are n=0 (an empty string) and n=1 ("0", "1", "8"). You recursively build up the strings until you reach the target length, ensuring that "00" is not added at the outermost layer to avoid leading zeros.
To find all strobogrammatic numbers of length n=2:
A common error is including "0" as a leading digit for numbers where n > 1. Another mistake is failing to handle the base cases for both n=0 and n=1 correctly, which are needed to build even and odd-length numbers respectively. Some candidates try to use an iterative approach with many nested loops, which becomes very difficult to manage compared to a clean recursive solution.
When tackling the Strobogrammatic Number II interview question, think of it as "building from the inside out." This perspective makes the recursion much easier to visualize. Also, pay close attention to the leading zero constraint—it's the most common reason for failing test cases. Be ready to explain the time complexity, which is related to the number of valid strobogrammatic digits (roughly 5^(n/2)).
| Title | Difficulty | Topics | LeetCode |
|---|---|---|---|
| Longest Common Prefix Between Adjacent Strings After Removals | Medium | Solve | |
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| Count Items Matching a Rule | Easy | Solve | |
| Check If Two String Arrays are Equivalent | Easy | Solve |
