Today I have done leetcode's November LeetCoding Challenge with cpp.

November LeetCoding Challenge 21

Description

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.

Solution

class Solution {
  int rows;
  int cols;
  void horizontalGuard(vector<vector<int>> &cells, int row, int col) {
    cells[row][col] |= 1;
    for(int i = col + 1; i < cols; ++i) {
      if(cells[row][i] == -1) break;
      cells[row][i] = (cells[row][i] | 1);
    }
    for(int i = col - 1; i >= 0; --i) {
      if(cells[row][i] == -1) break;
      cells[row][i] = (cells[row][i] | 1);
    }
  }
  void verticalGuard(vector<vector<int>> &cells, int row, int col) {
    cells[row][col] |= 2;
    for(int i = row + 1; i < rows; ++i) {
      if(cells[i][col] == -1) break;
      cells[i][col] = (cells[i][col] | 2);
    }
    for(int i = row - 1; i >= 0; --i) {
      if(cells[i][col] == -1) break;
      cells[i][col] = (cells[i][col] | 2);
    }
  }
public:
  int countUnguarded(int m, int n, vector<vector<int>>& guards, vector<vector<int>>& walls) {
    rows = m;
    cols = n;
    vector<vector<int>> cells(m, vector<int>(n));
    for(const auto &wall : walls) {
      cells[wall[0]][wall[1]] = -1;
    }

    for(const auto &guard : guards) {
      if(!(cells[guard[0]][guard[1]] & 1)) horizontalGuard(cells, guard[0], guard[1]);
      if(!(cells[guard[0]][guard[1]] & 2)) verticalGuard(cells, guard[0], guard[1]);
    }

    int answer = 0;
    for(const auto &r : cells) {
      for(const auto &c : r) {
        answer += !c;
      }
    }
    return answer;
  }
};