Solution.java

// github.com/RodneyShag

// Tips:
// - Instead of using a "boolean[][] visited" array, we alter our original grid
// - Dont create a 2-D "Point" or "Cell" class. It's not necessary.

static int maxRegion(int[][] grid) {
    final int rows = grid.length;
    final int cols = grid[0].length;
    int maxRegion = 0;
    
    for (int row = 0; row < rows; row++) {
        for (int col = 0; col < cols; col++) {
            /* Find the largest region from the current cell */
            if (grid[row][col] == 1) {
                int size  = findLargestRegion(grid, row, col, rows, cols);
                maxRegion = Math.max(maxRegion, size);
            }
        }
    }
    return maxRegion;
}

private static int findLargestRegion(int [][] grid, int row, int col, int rows, int cols) {
    /* Put boundary checks here (at top of recursive call), instead of before doing recursive call */
    if (row < 0 || row >= rows || col < 0 || col >= cols || grid == null || grid[row][col] == 0) {
        return 0;
    }

    grid[row][col] = 0; // we alter the original matrix here
    int size = 1;       // 1 accounts for our size
    
    /* Recursively search neighbors */
    for (int r = row - 1; r <= row + 1; r++) {
        for (int c = col - 1; c <= col + 1; c++) {
            size += findLargestRegion(grid, r, c, rows, cols);
        }
    }

    return size;
}