Count Unguarded Cells in the Grid LeetCode solution in C++

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class Solution {
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 public:
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  void dfs(vector<vector<int>> &grid, int x, int y, int m, int n, int dir) {
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    if (x < 0 || y < 0 || x >= m || y >= n) return;
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    if (grid[x][y] == 2 || grid[x][y] == 1) return;
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    grid[x][y] = 3;
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    if (dir == 1) {
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      dfs(grid, x + 1, y, m, n, dir);
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    } else if (dir == 2) {
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      dfs(grid, x, y + 1, m, n, dir);
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    } else if (dir == 3) {
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      dfs(grid, x - 1, y, m, n, dir);
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    } else {
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      dfs(grid, x, y - 1, m, n, dir);
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    }
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  }
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  int countUnguarded(int m, int n, vector<vector<int>> &guards, vector<vector<int>> &walls) {
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    vector<vector<int>> grid(m, vector<int>(n, 0));
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    // marking guards
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    for (int i = 0; i < guards.size(); i++) {
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      int x = guards[i][0];
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      int y = guards[i][1];
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      grid[x][y] = 1;
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    }
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    // marking walls
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    for (int i = 0; i < walls.size(); i++) {
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      int x = walls[i][0];
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      int y = walls[i][1];
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      grid[x][y] = 2;
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    }
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    // dfs in each of 4 directions
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    for (int i = 0; i < guards.size(); i++) {
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      int x = guards[i][0];
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      int y = guards[i][1];
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      dfs(grid, x + 1, y, m, n, 1);
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      dfs(grid, x, y + 1, m, n, 2);
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      dfs(grid, x - 1, y, m, n, 3);
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      dfs(grid, x, y - 1, m, n, 4);
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    }
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    long long int cnt = 0;
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    for (int i = 0; i < m; i++) {
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      for (int j = 0; j < n; j++) {
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        if (grid[i][j] == 0) cnt++;
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      }
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    }
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    return cnt;
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  }
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};