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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.

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 (Rust)

1. Solution

impl Solution {
    pub fn count_unguarded(m: i32, n: i32, guards: Vec<Vec<i32>>, walls: Vec<Vec<i32>>) -> i32 {
        let (m, n) = (m as usize, n as usize);
        let mut grid = vec![vec![0; n]; m];
        let mut ret = 0;

        for i in 0..guards.len() {
            grid[guards[i][0] as usize][guards[i][1] as usize] = 1;
        }
        for i in 0..walls.len() {
            grid[walls[i][0] as usize][walls[i][1] as usize] = 2;
        }

        for r in 0..m {
            let mut can_see = false;

            for c in 0..n {
                match grid[r][c] {
                    1 => can_see = true,
                    2 => can_see = false,
                    _ if can_see => grid[r][c] = 3,
                    _ => (),
                }
            }

            can_see = false;

            for c in (0..n).rev() {
                match grid[r][c] {
                    1 => can_see = true,
                    2 => can_see = false,
                    _ if can_see => grid[r][c] = 3,
                    _ => (),
                }
            }
        }

        for c in 0..n {
            let mut can_see = false;

            for r in 0..m {
                match grid[r][c] {
                    1 => can_see = true,
                    2 => can_see = false,
                    _ if can_see => grid[r][c] = 3,
                    _ => (),
                }
            }

            can_see = false;

            for r in (0..m).rev() {
                match grid[r][c] {
                    1 => can_see = true,
                    2 => can_see = false,
                    _ if can_see => grid[r][c] = 3,
                    _ => (),
                }

                if grid[r][c] == 0 {
                    ret += 1;
                }
            }
        }

        ret
    }
}