NumberOfIslands.java

class NumberOfIslands {
    public int numIslands(char[][] grid) {
        int numIslands = 0;
        for (int r = 0; r < grid.length; r++) {
            for (int c = 0; c < grid[r].length; c++) {
                if (grid[r][c] == '1') {
                    dfs(grid, r, c);
                    numIslands++;
                }
            }
        }
        return numIslands;
    }
    private void dfs(char[][] grid, int row, int col) {
        if (row < 0 || row >= grid.length 
            || col < 0 || col >= grid[row].length 
            || grid[row][col] == '0') {
            return;
        }
        grid[row][col] = '0';
        dfs(grid, row - 1, col);
        dfs(grid, row, col + 1);
        dfs(grid, row + 1, col);
        dfs(grid, row, col - 1);
    }
}