200: Number of Islands

python/number_of_islands.py

@@ -0,0 +1,24 @@
+# https://leetcode.com/problems/number-of-islands/description/
+# T: O(MN) where M is the number of rows and N is the number of columns.
+# S: O(MN) where M is the number of rows and N is the number of columns.
+
+def numIslands(self, grid):
+    if not grid:
+        return 0
+
+    count = 0
+    for i in range(len(grid)):
+        for j in range(len(grid[0])):
+            if grid[i][j] == '1':
+                self.dfs(grid, i, j)
+                count += 1
+    return count
+
+def dfs(self, grid, i, j):
+    if i<0 or j<0 or i>=len(grid) or j>=len(grid[0]) or grid[i][j] != '1':
+        return
+    grid[i][j] = '#'
+    self.dfs(grid, i+1, j)
+    self.dfs(grid, i-1, j)
+    self.dfs(grid, i, j+1)
+    self.dfs(grid, i, j-1)