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01 Matrix

Breadth-First Search·Medium

Given an m x n binary matrix mat, return the distance of the nearest 0 for each cell.

The distance between two adjacent cells is 1. Adjacency is 4-directional (up, down, left, right).

Approach. Multi-source BFS starting from every 0 simultaneously: each cell's distance is set the first time BFS reaches it.

Examples

Input: mat = [[0,0,0],[0,1,0],[0,0,0]]

Output: [[0,0,0],[0,1,0],[0,0,0]]

Input: mat = [[0,0,0],[0,1,0],[1,1,1]]

Output: [[0,0,0],[0,1,0],[1,2,1]]

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Tests

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Passed32 ms

First attempt — retry to track metric movement here.

Complete approach — pattern and complexity both named.

You correctly matched the sorted input requirement and implemented a linear-time scan using two pointers.

No divergence detected. The code correctly aligns with the stated approach.

Try solving related sliding window problems to build familiarity with two-pointer variants.

No signals fired — clean run, you stayed in flow.