2343 Count Unguarded Cells In The Grid

Count Unguarded Cells in the Grid

You are given two integers m and n representing a 0-indexed m x n grid. You are also given two 2D integer arrays guards and walls where guards[i] = [rowi, coli] and walls[j] = [rowj, colj] represent the positions of the ith guard and jth wall respectively.

A guard can see every cell in the four cardinal directions (north, east, south, or west) starting from their position unless obstructed by a wall or another guard. A cell is guarded if there is at least one guard that can see it.

Return* the number of unoccupied cells that are not guarded.*

 

Example 1:

**Input:** m = 4, n = 6, guards = [[0,0],[1,1],[2,3]], walls = [[0,1],[2,2],[1,4]]
**Output:** 7
**Explanation:** The guarded and unguarded cells are shown in red and green respectively in the above diagram.
There are a total of 7 unguarded cells, so we return 7.

Example 2:

**Input:** m = 3, n = 3, guards = [[1,1]], walls = [[0,1],[1,0],[2,1],[1,2]]
**Output:** 4
**Explanation:** The unguarded cells are shown in green in the above diagram.
There are a total of 4 unguarded cells, so we return 4.

 

Constraints:

1 <= m, n <= 105
2 <= m * n <= 105
1 <= guards.length, walls.length <= 5 * 104
2 <= guards.length + walls.length <= m * n
guards[i].length == walls[j].length == 2
0 <= rowi, rowj < m
0 <= coli, colj < n
All the positions in guards and walls are **unique**.

class Solution:
    def countUnguarded(self, m: int, n: int, guards: List[List[int]], walls: List[List[int]]) -> int:
        grid = [[0] * n for _ in range(m)]
        
        # Mark guards and walls
        for r, c in guards:
            grid[r][c] = 1  # Guard
        for r, c in walls:
            grid[r][c] = 2  # Wall
        
        # Directions for guard coverage: up, down, left, right
        directions = [(-1, 0), (1, 0), (0, -1), (0, 1)]
        
        # Expand guard coverage
        for r, c in guards:
            for dr, dc in directions:
                nr, nc = r + dr, c + dc
                while 0 <= nr < m and 0 <= nc < n and grid[nr][nc] != 2 and grid[nr][nc] != 1:
                    grid[nr][nc] = 3  # Guarded
                    nr, nc = nr + dr, nc + dc
        
        # Count unguarded cells
        return sum(cell == 0 for row in grid for cell in row)