README_EN.md

2257. Count Unguarded Cells in the Grid

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Description

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.

Solutions

Python3

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)

Java

class Solution {
    public int countUnguarded(int m, int n, int[][] guards, int[][] walls) {
        char[][] g = new char[m][n];
        for (int[] e : guards) {
            int r = e[0], c = e[1];
            g[r][c] = 'g';
        }
        for (int[] e : walls) {
            int r = e[0], c = e[1];
            g[r][c] = 'w';
        }
        int[][] dirs = new int[][] {{0, -1}, {0, 1}, {1, 0}, {-1, 0}};
        for (int[] p : guards) {
            for (int[] dir : dirs) {
                int a = dir[0], b = dir[1];
                int x = p[0], y = p[1];
                while (x + a >= 0 && x + a < m && y + b >= 0 && y + b < n && g[x + a][y + b] != 'w'
                    && g[x + a][y + b] != 'g') {
                    x += a;
                    y += b;
                    g[x][y] = 'v';
                }
            }
        }
        int ans = 0;
        for (char[] row : g) {
            for (char v : row) {
                if (v == 0) {
                    ++ans;
                }
            }
        }
        return ans;
    }
}

C++

class Solution {
public:
    int countUnguarded(int m, int n, vector<vector<int>>& guards, vector<vector<int>>& walls) {
        vector<vector<char>> g(m, vector<char>(n));
        for (auto& e : guards) g[e[0]][e[1]] = 'g';
        for (auto& e : walls) g[e[0]][e[1]] = 'w';
        vector<vector<int>> dirs = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
        for (auto& p : guards) {
            for (auto& dir : dirs) {
                int a = dir[0], b = dir[1];
                int x = p[0], y = p[1];
                while (x + a >= 0 && x + a < m && y + b >= 0 && y + b < n && g[x + a][y + b] != 'w' && g[x + a][y + b] != 'g') {
                    x += a;
                    y += b;
                    g[x][y] = 'v';
                }
            }
        }
        int ans = 0;
        for (auto& row : g)
            for (auto& v : row)
                ans += v == 0;
        return ans;
    }
};

Go

func countUnguarded(m int, n int, guards [][]int, walls [][]int) int {
	g := make([][]int, m)
	for i := range g {
		g[i] = make([]int, n)
	}
	for _, e := range guards {
		g[e[0]][e[1]] = 1
	}
	for _, e := range walls {
		g[e[0]][e[1]] = 2
	}
	dirs := [][]int{{0, -1}, {0, 1}, {1, 0}, {-1, 0}}
	for _, p := range guards {
		for _, dir := range dirs {
			a, b := dir[0], dir[1]
			x, y := p[0], p[1]
			for x+a >= 0 && x+a < m && y+b >= 0 && y+b < n && g[x+a][y+b] != 1 && g[x+a][y+b] != 2 {
				x, y = x+a, y+b
				g[x][y] = 3
			}
		}
	}
	ans := 0
	for _, row := range g {
		for _, v := range row {
			if v == 0 {
				ans++
			}
		}
	}
	return ans
}

TypeScript

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