LeetCode 542 01 Matrix - Problem Statement

You have a binary matrix where each cell is either $0$ or $1$. For each cell, find the distance to the nearest $0$.

The distance between two adjacent cells is $1$. You can only move up, down, left, or right.

Return a matrix of the same size where each cell contains the distance to the nearest $0$.

This is another multi-source BFS problem. Instead of spreading from one source, you spread from all $0$ cells at once. Before reading on, think: why start from $0$ cells instead of $1$ cells? What would happen if you ran BFS from each $1$ cell individually?