Number of Islands LeetCode solution in Java

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class Solution {
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  public int numIslands(char[][] grid) {
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    int count = 0;
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    int r = grid.length;
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    int c = grid[0].length;
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    for (int i = 0; i < r; i++) {
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      for (int j = 0; j < c; j++) {
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        if (grid[i][j] == '1') {
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          dfs(grid, i, j);
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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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  public void dfs(char[][] grid, int i, int j) {
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    if (i < 0 || j < 0 || i >= grid.length || j >= grid[0].length || grid[i][j] == '0') {
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      return;
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    }
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    grid[i][j] = '0';
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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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}