Walls And Gates

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Problem

Given a grid of rooms where -1 represents a wall, 0 represents a gate, and a large sentinel value represents an empty room, fill each empty room with the distance to its nearest gate. Leave rooms that can't reach a gate unchanged.

Examples

Example 1

Input:grid with a couple of gates and several empty rooms

Output:each empty room filled with its distance to the nearest gate

Constraints

1 <= m, n <= 250

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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 ↗