LeetCode 1091 Shortest Path in Binary Matrix - Implementation Details

In grid BFS, nodes are coordinate pairs $(r, c)$. Here is the pattern:

function shortestPathBinaryMatrix(grid):
    n = grid.size
    if grid[0][0] == 1 or grid[n-1][n-1] == 1:
        return -1

    dist = 2D array of size n x n, all -1
    dist[0][0] = 1
    q = queue with (0, 0)

    directions = [(-1,-1),(-1,0),(-1,1),(0,-1),(0,1),(1,-1),(1,0),(1,1)]

    while q is not empty:
        (r, c) = q.pop()
        if r == n - 1 and c == n - 1:
            return dist[r][c]
        for (dr, dc) in directions:
            nr = r + dr
            nc = c + dc
            if 0 <= nr < n and 0 <= nc < n and grid[nr][nc] == 0 and dist[nr][nc] == -1:
                dist[nr][nc] = dist[r][c] + 1
                q.push((nr, nc))

    return -1

This runs in $O(n^2)$ time and uses $O(n^2)$ space.