What is the optimal time complexity of solving Surrounded Regions?

The optimized solution achieves a time complexity of O(n) by using sorting, mathematical shortcuts, or hash table lookups.

What is the auxiliary space complexity for Surrounded Regions?

The space complexity is O(1) since we only use a minimal/constant amount of extra memory for tracking variables (or auxiliary space if dynamic structures are allocated).

What are the typical edge cases we must handle in Surrounded Regions?

Key edge cases include empty collections, negative or large bounds causing integer overflow, arrays with only one element, or inputs where target criteria are not met.

How can we optimize the brute force approach for Surrounded Regions?

Brute force methods often inspect all possible combinations. We optimize this by sorting the input list to enable binary search, or tracking processed items in a set to avoid redundant nested loop lookups.

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Surrounded Regions

Learn how to solve the 'Surrounded Regions' problem. This detailed resource details brute force and optimized approaches.

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Problem Statement

Easy

Given an m x n matrix board containing 'X' and 'O', capture all regions that are 4-directionally surrounded by 'X'.

A region is captured by flipping all 'O's into 'X's in that surrounded region.

Write a function solve(board: List[List[str]]) -> List[List[str]] that returns the modified board.

Constraints

•m == len(board)
•n == len(board[i])
•1 <= m, n <= 200
•board[i][j] is 'X' or 'O'

Examples

Example 1

Input

board = [["X","X","X","X"],["X","O","O","X"],["X","X","O","X"],["X","O","X","X"]]

Output

[["X","X","X","X"],["X","X","X","X"],["X","X","X","X"],["X","O","X","X"]]

Explanation

Surrounded regions should not be on the border, which means any 'O' on the border of the board is not flipped to 'X'. Any 'O' that is connected to a border 'O' is also not flipped.

Need a Hint?

Represent graph node connections as an adjacency list/matrix, then use standard BFS or DFS graph traversal.

Edge Cases to Watch

Empty list or null input variables
Single item lists/arrays
Extremely large input bounds causing integer or stack overflow

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