Number of Closed Islands LeetCode solution in Java

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class Solution {
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  boolean isClosed = true;
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  public int closedIsland(int[][] grid) {
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    int m = grid.length;
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    int n = grid[0].length;
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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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        isClosed = true;
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        if (grid[i][j] == 0) {
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          dfs(grid, i, j);
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          if (isClosed) {
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            count++;
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          }
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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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  public void dfs(int[][] grid, int i, int j) {
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    if (i < 0 || j < 0 || i > grid.length - 1 || j > grid[0].length - 1 || grid[i][j] != 0) return;
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    grid[i][j] = 1; // to mark as visited
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    if (i == 0 || j == 0 || i == grid.length - 1 || j == grid[0].length - 1) isClosed = false;
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    dfs(grid, i, j + 1);
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    dfs(grid, i, j - 1);
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    dfs(grid, i + 1, j);
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    dfs(grid, i - 1, j);
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  }
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}