Solve "count islands" problem

This morning, I attended the "Interview Club" and was asked this question by the
interviewer in front of ~20 FTEs. While I struggled to fully solve it during the
abridged (i.e. 20 minute) timeslot, I completed the problem afterwards.

Here is my solution.

1 files changed, 53 insertions, 0 deletions

diff --git a/scratch/facebook/count-islands.py b/scratch/facebook/count-islands.py
new file mode 100644
index 000000000000..a007684bca23
--- /dev/null
+++ b/scratch/facebook/count-islands.py
@@ -0,0 +1,53 @@
+from collections import deque
+
+def maybe_queue(row, col, game, q, seen):
+    """
+    Add coordinate, (`row`, `col`), to the queue, `q`, as long as it exists in
+    the map, `game`, and it is not already present in `seen`.
+    """
+    if row >= 0 and row < len(game) and col >= 0 and col < len(game[0]):
+        if game[row][col] == 'L' and (row, col) not in seen:
+            q.append((row, col))
+
+def visit_island(row, col, game, seen):
+    """
+    Starting at the coordinate, (`row`, `col`), in the map, `game`, visit all
+    surrounding tiles marked as land by adding them to the `seen` set.
+    """
+    q = deque()
+    q.append((row, col))
+    while q:
+        row, col = q.popleft()
+        seen.add((row, col))
+        maybe_queue(row - 1, col, game, q, seen) # UP
+        maybe_queue(row + 1, col, game, q, seen) # DOWN
+        maybe_queue(row, col - 1, game, q, seen) # LEFT
+        maybe_queue(row, col + 1, game, q, seen) # RIGHT
+
+def count_islands(game):
+    """
+    Return the number of contiguous land tiles in the map, `game`.
+    """
+    result = 0
+    seen = set()
+    for row in range(len(game)):
+        for col in range(len(game[row])):
+            if game[row][col] == 'L' and (row, col) not in seen:
+                visit_island(row, col, game, seen)
+                result += 1
+    return result
+
+################################################################################
+# Tests
+################################################################################
+
+game = [
+    "LWLWWW",
+    "LLLWWW",
+    "WWWLLW",
+]
+
+result = count_islands(game)
+print(result)
+assert result == 2
+print("Success!")