Number of Closed Islands LeetCode solution in C++

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class Solution {
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 public:
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  void dfs(int i, int j, vector<vector<int>> &grid) {
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    int m = grid.size(), n = grid[0].size();
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    if (i < 0 || i >= m || j < 0 || j >= n || grid[i][j] != 0) return;
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    grid[i][j] = 1;
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    int dx[4] = {1, -1, 0, 0};
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    int dy[4] = {0, 0, 1, -1};
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    for (int k = 0; k < 4; k++) {
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      int nx = i + dx[k];
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      int ny = j + dy[k];
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      dfs(nx, ny, grid);
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    }
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  }
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  int closedIsland(vector<vector<int>> &grid) {
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    int m = grid.size(), n = grid[0].size();
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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 ((i * j == 0 || i == m - 1 || j == n - 1) && (grid[i][j] == 0)) dfs(i, j, grid);
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      }
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    }
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    int count = 0;
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    for (int i = 1; i < m - 1; i++) {
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      for (int j = 1; j < n - 1; j++) {
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        if (grid[i][j] == 0) {
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          dfs(i, j, grid);
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          count++;
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        }
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      }
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    }
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    return count;
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  }
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};