2257. Count Unguarded Cells in the Grid

Problem Statement

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 guards[i] = [row_i, col_i] and walls[j] = [row_j, col_j] represent the positions of the i^th guard and j^th 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 <= row_i, row_j < m
0 <= col_i, col_j < n
All the positions in guards and walls are unique.

Java

Source file

class Solution {
    private static int[][] dir = { { 0, 1 }, { 1, 0 }, { 0, -1 }, { -1, 0 } };

    public int countUnguarded(int m, int n, int[][] guards, int[][] walls) {
        int count = 0;
        int[][] grid = new int[m][n];
        for (int[] w : walls)
            grid[w[0]][w[1]] = 1; // blocked
        for (int[] g : guards)
            grid[g[0]][g[1]] = 1; // blocked
        for (int[] g : guards) {
            grid[g[0]][g[1]] = 2; // unblock temp.
            for (int[] d : dir) {
                for (int i = g[0], j = g[1]; i >= 0 && i < m && j >= 0 && j < n
                        && grid[i][j] != 1; i += d[0], j += d[1]) {
                    if (grid[i][j] == 0) // new
                        count++;
                    grid[i][j] = 2; // visited
                }
            }
            grid[g[0]][g[1]] = 1; // block back
        }
        // System.out.println(count);
        return m * n - count - guards.length - walls.length;
    }
}