2257. 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.

class Solution {
public:
    int countUnguarded(int n, int m, vector<vector<int>>& guards, vector<vector<int>>& walls) {
        vector<vector<int>> g(n, vector<int>(m, 0));
        int dy[4] {-1,0,1,0}, dx[4]{0,1,0,-1};
        for(auto& p : guards) {
            g[p[0]][p[1]] = -1;
        }
        for(auto& p : walls) {
            g[p[0]][p[1]] = -2;
        }
        for(auto& p : guards) {
            int y = p[0], x = p[1];
            for(int i = 0; i < 4; i++) {
                int ny = y + dy[i], nx = x + dx[i];
                while(0 <= ny and ny < n and 0 <= nx and nx < m) {
                    if(g[ny][nx] < 0) break;
                    if(g[ny][nx] & (1<<((i + 2) % 4))) break;
                    g[ny][nx] |= (1<<i);
                    ny += dy[i], nx += dx[i];
                }
            }
        }
        
        int res = 0;
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < m; j++) {
                if(g[i][j] == 0) res++;
            }
        }
        return res;
    }
};