2257 - Count Unguarded Cells in the Grid.

Medium

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 <code>guardsi = row_i, col_i</code> and <code>wallsj = row_j, col_j</code> represent the positions of the <code>i^th</code> guard and <code>j^th</code> 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:

• <code>1 <= m, n <= 10⁵</code>

• <code>2 <= m * n <= 10⁵</code>

• <code>1 <= guards.length, walls.length <= 5 * 10⁴</code>

• 2 <= guards.length + walls.length <= m * n

• guards[i].length == walls[j].length == 2

• <code>0 <= row_i, row_j< m</code>

• <code>0 <= col_i, col_j< n</code>

• All the positions in guards and walls are unique.