Leetcode 130. Surrounded Regions

Given an m x n board of 'X' and 'O', flip all 'O' cells that belong to regions completely surrounded by 'X' to 'X' (regions connected to the border remain unchanged). The core challenge is to identify border-connected 'O' regions (e.g., via DFS/BFS or Union-Find) and flip all other 'O's in-place.