Formatted question description: https://leetcode.ca/all/2257.html

2257. Count Unguarded Cells in the Grid

Difficulty: Medium.
Related Topics: Array, Matrix, Simulation.
Similar Questions: Bomb Enemy, Available Captures for Rook.

Problem

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.

Solution (Java, C++, Python)

class Solution {
    public int countUnguarded(int m, int n, int[][] guards, int[][] walls) {
        char[][] matrix = new char[m][n];
        int result = 0;
        for (int i = 0; i <= guards.length - 1; i++) {
            int guardRow = guards[i][0];
            int guardCol = guards[i][1];
            matrix[guardRow][guardCol] = 'G';
        }
        for (int i = 0; i <= walls.length - 1; i++) {
            int wallRow = walls[i][0];
            int wallCol = walls[i][1];
            matrix[wallRow][wallCol] = 'W';
        }
        for (int i = 0; i <= guards.length - 1; i++) {
            int currentRow = guards[i][0];
            int currentCol = guards[i][1] - 1;
            while (currentCol >= 0) {
                if (matrix[currentRow][currentCol] != 'W'
                        && matrix[currentRow][currentCol] != 'G') {
                    matrix[currentRow][currentCol] = 'R';
                } else {
                    break;
                }
                currentCol--;
            }
            currentRow = guards[i][0];
            currentCol = guards[i][1] + 1;
            while (currentCol <= n - 1) {
                if (matrix[currentRow][currentCol] != 'W'
                        && matrix[currentRow][currentCol] != 'G') {
                    matrix[currentRow][currentCol] = 'R';
                } else {
                    break;
                }
                currentCol++;
            }
            currentRow = guards[i][0] - 1;
            currentCol = guards[i][1];
            while (currentRow >= 0) {
                if (matrix[currentRow][currentCol] != 'W'
                        && matrix[currentRow][currentCol] != 'G') {
                    matrix[currentRow][currentCol] = 'R';
                } else {
                    break;
                }
                currentRow--;
            }
            currentRow = guards[i][0] + 1;
            currentCol = guards[i][1];
            while (currentRow <= m - 1) {
                if (matrix[currentRow][currentCol] != 'W'
                        && matrix[currentRow][currentCol] != 'G') {
                    matrix[currentRow][currentCol] = 'R';
                } else {
                    break;
                }
                currentRow++;
            }
        }
        for (int i = 0; i <= m - 1; i++) {
            for (int j = 0; j <= n - 1; j++) {
                if (matrix[i][j] != 'R' && matrix[i][j] != 'G' && matrix[i][j] != 'W') {
                    result++;
                }
            }
        }
        return result;
    }
}

```
Todo
```

class Solution:
    def countUnguarded(
        self, m: int, n: int, guards: List[List[int]], walls: List[List[int]]
    ) -> int:
        g = [[None] * n for _ in range(m)]
        for r, c in guards:
            g[r][c] = 'g'
        for r, c in walls:
            g[r][c] = 'w'
        for i, j in guards:
            for a, b in [[0, -1], [0, 1], [1, 0], [-1, 0]]:
                x, y = i, j
                while (
                    0 <= x + a < m
                    and 0 <= y + b < n
                    and g[x + a][y + b] != 'w'
                    and g[x + a][y + b] != 'g'
                ):
                    x, y = x + a, y + b
                    g[x][y] = 'v'
        return sum(not v for row in g for v in row)

############

# 2257. Count Unguarded Cells in the Grid
# https://leetcode.com/problems/count-unguarded-cells-in-the-grid

class Solution:
    def countUnguarded(self, rows: int, cols: int, guards: List[List[int]], walls: List[List[int]]) -> int:
        grid = [[1] * cols for _ in range(rows)]
        
        for x, y in walls:
            grid[x][y] = -1
            
        def goLeft(x, y):
            dy = y - 1
            while dy >= 0 and grid[x][dy] != 2 and grid[x][dy] != -1:
                grid[x][dy] = 2
                dy -= 1
        
        def goRight(x, y):
            dy = y + 1
            while dy < cols and grid[x][dy] != 3 and grid[x][dy] != -1:
                grid[x][dy] = 3
                dy += 1
                
        def goTop(x, y):
            dx = x - 1
            while dx >= 0 and grid[dx][y] != 4 and grid[dx][y] != -1:
                grid[dx][y] = 4
                dx -= 1
                
        def goBottom(x, y):
            dx = x + 1
            while dx < rows and grid[dx][y] != 5 and grid[dx][y] != -1:
                grid[dx][y] = 5
                dx += 1
    
        guards.sort(key = lambda x : (-x[1], x[0]))
​
        for x, y in guards:
            goLeft(x, y)
​
        guards.sort(key = lambda x : (x[1], x[0]))
​
        for x, y in guards:
            goRight(x, y)
        
        guards.sort(key = lambda x : (-x[0], x[1]))
        for x, y in guards:
            goTop(x, y)
​
        guards.sort(key = lambda x : (x[0], x[1]))
        for x, y in guards:
            goBottom(x, y)
​
        for x, y in guards:
            grid[x][y] = -1
        
        res = 0
        
        for x in range(rows):
            for y in range(cols):
                if grid[x][y] == 1:
                    res += 1
        
        return res

Explain:

nope.

Complexity:

Time complexity : O(n).
Space complexity : O(n).

2257 - Count Unguarded Cells in the Grid

2257. Count Unguarded Cells in the Grid

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2257. Count Unguarded Cells in the Grid

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Solution (Java, C++, Python)

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