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Problem 17: Territorial Enclosure (Surrounded Regions)

Problem Statement

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.

Input Format

  • A 2D character matrix board.

Example

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: The 'O' at the bottom edge is not surrounded, so it stays. All other 'O's are flipped.