Shortest Path Between Two Points in a 2D Matrix

Question

## Problem

Given a grid with blocked cells, find the shortest path length between two points (4-directional moves).

## Example
```
BFS from source until target reached
```

## Constraints
- 1 ≤ m,n ≤ 500

## Follow-up
Weighted cells -> Dijkstra; allow breaking up to k walls -> 3D BFS state.