Is Generate Binary Strings Without Adjacent Zeros easy or hard?

The problem is typically classified as Medium. The constraint itself is simple, but recognizing that you should prune invalid branches using backtracking is the key insight. Once that idea is clear, the implementation is straightforward.

Generate Binary Strings Without Adjacent Zeros Python/Java solution

In Python or Java, implement a backtracking function that appends '1' always and appends '0' only if the previous character is not '0'. When the constructed string length reaches n, add it to the result list. This approach works the same across C++, JavaScript, C#, and Python.

How to solve Generate Binary Strings Without Adjacent Zeros in O(n)?

Generating all valid strings cannot be done in O(n) time because the output itself grows exponentially. However, counting the number of valid strings can be solved with dynamic programming in O(n) time using the recurrence similar to Fibonacci: dp[i] = dp[i-1] + dp[i-2].

What is the best approach for Generate Binary Strings Without Adjacent Zeros?

Backtracking is the most common and expected approach. Build the string one character at a time and only place '0' if the previous character is not '0'. This pruning guarantees that invalid "00" patterns never appear. The time complexity is O(2^n) in the worst case with O(n) recursion stack space.

Is Generate Binary Strings Without Adjacent Zeros asked at Google/Amazon/Meta?

Variants of this constraint-generation problem appear in interviews at companies like Google, Amazon, and Meta. The pattern tests understanding of backtracking, recursion, and pruning invalid states early. It is also related to Fibonacci-style DP counting problems.

What data structure is used in Generate Binary Strings Without Adjacent Zeros?

Most implementations use recursion with a mutable string or character array while building candidates. A list or vector stores the final results. Some iterative solutions maintain two lists representing strings ending in '0' and '1'.

What is the time complexity of Generate Binary Strings Without Adjacent Zeros?

The generation cost is proportional to the number of valid strings. In the worst case it is bounded by O(2^n), but the actual count follows Fibonacci growth because strings with "00" are pruned. The recursion stack or temporary string requires O(n) auxiliary space.

Description Hints 3 Solution Video 8:30

3211. Generate Binary Strings Without Adjacent Zeros

Medium88.3% acceptance4 companies

You are given a positive integer n.

A binary string x is valid if all substrings of x of length 2 contain at least one "1".

Return all valid strings with length n, in any order.

Example 1:

Input: n = 3

Output: ["010","011","101","110","111"]

Explanation:

The valid strings of length 3 are: "010", "011", "101", "110", and "111".

Example 2:

Input: n = 1

Output: ["0","1"]

Explanation:

The valid strings of length 1 are: "0" and "1".

Constraints:

1 <= n <= 18

Asked at 4 companies

Amazon

Google Microsoft Bloomberg

Topics

Hints open one at a time

Hint 1

If we have a string s of length x, we can generate all strings of length x + 1.

Hint 2

If s has 0 as the last character, we can only append 1, whereas if the last character is 1, we can append both 0 and 1.

Hint 3

We can use recursion and backtracking to generate all such strings.

8:30

Still stuck?Read 3 approaches, or watch a walkthrough.

Open solution