Add solution for challenge #54

Author: Mauricio Klein

README.md

@@ -53,3 +53,4 @@
 - [x] [Challenge 51: Subsequences of string](challenge-51/)
 - [x] [Challenge 52: Deepest Node in a Binary Tree](challenge-52/)
 - [x] [Challenge 53: Thousand missing numbers](challenge-53/)
+- [x] [Challenge 54: Find the number of islands](challenge-54/)

challenge-54/Makefile

@@ -0,0 +1,4 @@
+test:
+	python test_*.py
+
+.PHONY: test

challenge-54/README.md

@@ -0,0 +1,26 @@
+# Find the number of islands
+
+This problem was asked by Linkedin.
+
+## Description
+
+Given a 2-dimensional grid consisting of 1's (land blocks) and 0's (water blocks), count the number of islands present in the grid. 
+
+The definition of an island is as follows:
+
+1. Must be surrounded by water blocks.
+2. Consists of land blocks (1's) connected to adjacent land blocks (either vertically or horizontally).
+
+Assume all edges outside of the grid are water.
+
+## Example
+
+```
+Input:
+  10001
+  11000
+  10110
+  00000
+
+Output: 3
+```

challenge-54/__init__.py

@@ -0,0 +1,40 @@
+
+#
+# Time complexity : O(n*m) (where n = rows in the grid, m = columns in the grid)
+# Space complexity: O(n*m) (worst case, when all the positions are land and the walk goes recursively to all positions)
+#
+def count_islands(grid):
+  rows, columns = len(grid), len(grid[0])
+  islands = 0
+
+  for row in range(rows):
+    for column in range(columns):
+      if grid[row][column] == 1:
+        islands += 1
+        walk(grid, (row, column), (rows, columns))
+
+  return islands
+
+#
+# walk marks the cell (pos) + all non-visited neighbours
+# as visited (cell value = 2)
+#
+def walk(grid, pos, dimensions):
+  r, c = pos
+  rows, columns = dimensions
+
+  def inbounds(row, column):
+    return (0 <= row < rows) and (0 <= column < columns)
+
+  # Mark the current position as visited
+  grid[r][c] = 2
+
+  # Mark the non-visited land neighbours as visited
+  paths = [(-1,0), (1,0), (0,-1), (0,1)]
+
+  for path in paths:
+    nr = r + path[0]
+    nc = c + path[1]
+
+    if inbounds(nr, nc) and grid[nr][nc] == 1:
+      walk(grid, (nr, nc), dimensions)

challenge-54/solver.py

@@ -0,0 +1,55 @@
+import unittest, random
+from solver import count_islands
+
+class TestSolver(unittest.TestCase):
+  def test_count_islands(self):
+    grid = [
+      [1,0,0,0,1],
+      [1,1,0,0,0],
+      [1,0,1,1,0],
+      [0,0,0,0,0]
+    ]
+    self.assertEqual(count_islands(grid), 3)
+
+    grid = [
+      [1,0,0,0,1],
+      [1,1,0,1,1],
+      [1,1,0,1,1],
+      [1,0,0,0,1]
+    ]
+    self.assertEqual(count_islands(grid), 2)
+
+    grid = [
+      [1,0,0,0,1],
+      [0,1,0,1,0],
+      [0,1,0,1,0],
+      [1,0,0,0,1]
+    ]
+    self.assertEqual(count_islands(grid), 6)
+
+    grid = [
+      [0,0,0,0,0],
+      [0,1,1,1,0],
+      [0,1,1,1,0],
+      [0,0,0,0,0]
+    ]
+    self.assertEqual(count_islands(grid), 1)
+
+    grid = [
+      [0,0,0,0,0],
+      [0,0,0,0,0],
+      [0,0,0,0,0],
+      [0,0,0,0,0]
+    ]
+    self.assertEqual(count_islands(grid), 0)
+
+    grid = [
+      [1,1,1,1,1],
+      [1,1,1,1,1],
+      [1,1,1,1,1],
+      [1,1,1,1,1]
+    ]
+    self.assertEqual(count_islands(grid), 1)
+
+if __name__ == "__main__":
+    unittest.main()