Surrounded Regions

Asked byGoogleBloombergUbertiktok

Problem

Given an m x n board of 'X' and 'O' characters, capture every region of 'O' that is completely surrounded by 'X' — meaning it has no connection to an 'O' on the border — by flipping it to 'X', in place.

Examples

Example 1

Input:board with an inner O-region fully enclosed by X

Output:the enclosed region is flipped to X; border-connected O's stay

Constraints

1 <= m, n <= 200

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How to approach it: the Graphs pattern

A graph models relationships as nodes and edges, which can be directed or undirected, weighted or not. Most graph problems come down to choosing the right traversal — DFS, BFS, or union-find — for what you're trying to find.

Look for this pattern when

The problem involves connections between items (islands, courses with prerequisites, friend networks, grids).
You need the shortest path in an unweighted graph (BFS) or need to explore every reachable node (DFS).
You're asking "are these two things connected?" repeatedly (union-find).

Read the full Graphs guide →

Video walkthroughs

Original problem on LeetCode ↗