Is Longest Balanced Substring After One Swap easy or hard?

The problem is rated Medium because the brute force idea is straightforward but the optimal solution requires reasoning about imbalance positions and how a single swap affects substring validity. Recognizing the greedy linear scan is the key insight.

Longest Balanced Substring After One Swap Python/Java solution

Python, Java, and C++ implementations typically follow the same idea: iterate through the string, track balance counts, detect invalid segments, and compute the maximum length achievable after correcting one imbalance with a swap. The final solution runs in O(n) time.

How to solve Longest Balanced Substring After One Swap in O(n)?

Perform a single pass through the string while maintaining a balance counter and tracking invalid boundaries. Identify segments where one misplaced character causes imbalance. A single swap can fix one such segment, allowing two valid regions to merge into a longer balanced substring.

What is the best approach for Longest Balanced Substring After One Swap?

The optimal method uses a greedy scan with prefix balance tracking. By analyzing where the string becomes imbalanced and how a single swap can fix one mismatch block, you can compute the maximum achievable balanced substring in O(n) time and O(1) space.

Is Longest Balanced Substring After One Swap asked at Google/Amazon/Meta?

Problems involving longest valid substrings, balance counters, and single modification operations appear frequently in interviews at companies like Google, Amazon, and Meta. Variants of longest valid parentheses and substring correction are common string algorithm questions.

What data structure is used in Longest Balanced Substring After One Swap?

The solution mainly uses counters and prefix balance tracking rather than complex data structures. Some implementations also rely on stack logic similar to longest valid parentheses problems or prefix arrays for faster imbalance detection.

What is the time complexity of Longest Balanced Substring After One Swap?

The optimal solution runs in O(n) time with O(1) extra space using a single linear scan and balance counters. Brute force approaches that try every swap pair take O(n^3), while improved simulations with prefix balance checks reduce it to about O(n^2).

3900. Longest Balanced Substring After One Swap - Practice Online

You are given a binary string s consisting only of characters '0' and '1'.

A string is balanced if it contains an equal number of '0's and '1's.

You can perform at most one swap between any two characters in s. Then, you select a balanced substring from s.

Return an integer representing the maximum length of the balanced substring you can select.

Example 1:

Input: s = "100001"

Output: 4

Explanation:

Swap "100001". The string becomes "101000".
Select the substring "101000", which is balanced because it has two '0's and two '1's.

Example 2:

Input: s = "111"

Output: 0

Explanation:

Choose not to perform any swaps.
Select the empty substring, which is balanced because it has zero '0's and zero '1's.

Constraints:

1 <= s.length <= 10⁵
s consists only of the characters '0' and '1'.