Find shortest path in a grid with obstacles

You are given a 2D grid of size `m x n` representing a maze. Each cell in the grid is either empty (0) or blocked (1). You are also given two coordinates: - `start = (sx, sy)` - `end = (ex, ey)` You can move from a cell to its 4-directional neighbors (up, down, left, right), but you **cannot** move into or through blocked cells, and you must remain inside the grid. Return the length of the shortest path (in number of steps) from `start` to `end`. If there is no valid path, return `-1`. Assumptions and details: - `0 <= sx, ex < m` - `0 <= sy, ey < n` - `grid[sx][sy] == 0` and `grid[ex][ey] == 0` - `1 <= m, n <= 10^3` - Time complexity should be efficient for the upper bounds (you may consider BFS or DFS-based approaches). **Input format (for reference):** - `m, n` (integers) - `grid` as an `m x n` matrix of 0s and 1s - `start` coordinate - `end` coordinate **Output:** - An integer representing the length of the shortest path, or `-1` if unreachable.