solution to LeetCode 200 Number of Islands via bfs

200_islands_bfs_noVisitedSet.py

+# -*- coding: utf-8 -*-
+"""
+Created on Fri Apr 15 11:04:07 2022
+
+@author: John Bedingfield
+"""
+
+
+# note: this solution was accepted for LeetCode #200: Number of Islands
+# Runtime: 451 ms, faster than 39.83% of python3 submissions
+# Memory Usage: 16.4MB, less than 62.10% of python3 submissions
+
+from collections import deque     # need a queue for bfs
+from time import time             # to capture timing data
+
+
+# ====== start pasting from here ======= 
+
+class Solution:
+    def numIslands(self, grid) -> int:
+        rows = len(grid)                # number of rows
+        cols = len(grid[0])             # number of cols
+        numOfIslands = 0                # variable for total number of islands
+        # helper arrays to calculate: up, down, right, left
+        dX = [-1, 0, 1, 0]  # rows
+        dY = [0, 1, 0, -1]  # cols
+        
+        ''' Breadth First Search:  takes (row, col) and completes a breadth first search
+            replaces the "1"s with "0"s as it goes (Carlos's idea),
+              (this eliminates the need to create a 'visited' set
+              to keep track of elements already visited)
+        '''
+        def bfs(x, y):
+            q = deque()                 # bfs accomplished by using a queue (FIFO)
+            q.append((x,y))             # add this element to the queue
+            while q:                    # while there are still elements to be searched
+                thisOne = q.popleft()                       # get the next one from the queue
+                thisX, thisY = thisOne[0], thisOne[1]       # get x and y
+                if grid[thisX][thisY] == "1":               # ignore dupes in the queue
+                    grid[thisX][thisY] = "0"                # mark as visited
+                    
+                    # check up, right, down, left from thisOne
+                    for delta in range(4):
+                        nextOneX = thisX + dX[delta]        # calc new x (row)
+                        nextOneY = thisY + dY[delta]        # calc new y (col)
+                        # check to ensure it's not already visited, it's a 1, and it's inbounds
+                        if (nextOneX in range(rows) and 
+                            nextOneY in range(cols) and
+                            grid[nextOneX][nextOneY] == "1"):
+                                # if so, it's part of the island, we want to search it, add to queue
+                                q.append((nextOneX, nextOneY))  
+            return            # end of bfs
+        
+        
+        ''' loop through each element in the grid,
+           if it's a "1" (land), it's a new island
+           increase the count, and conduct bfs to determine
+           the contours of the island
+        '''
+        for r in range(rows):
+            for c in range(cols):
+                if grid[r][c] == "1":
+                    bfs(r,c)
+                    numOfIslands = numOfIslands + 1
+                
+        return numOfIslands    # end numIslands
+
+                        
+# ====== stop pasting here =======
+                
+        
+        
+
+if __name__ == "__main__":
+    grid1 = [
+  ["1","1","1","1","0"],
+  ["1","1","0","1","0"],
+  ["1","1","0","0","0"],
+  ["0","0","0","0","0"]
+]
+    
+    
+    grid2 = [["1","1","1","1","1","0","1","1","1","1","1","1","1","1","1","0","1","0","1","1"],["0","1","1","1","1","1","1","1","1","1","1","1","1","0","1","1","1","1","1","0"],["1","0","1","1","1","0","0","1","1","0","1","1","1","1","1","1","1","1","1","1"],["1","1","1","1","0","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1"],["1","0","0","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1"],["1","0","1","1","1","1","1","1","0","1","1","1","0","1","1","1","0","1","1","1"],["0","1","1","1","1","1","1","1","1","1","1","1","0","1","1","0","1","1","1","1"],["1","1","1","1","1","1","1","1","1","1","1","1","0","1","1","1","1","0","1","1"],["1","1","1","1","1","1","1","1","1","1","0","1","1","1","1","1","1","1","1","1"],["1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1"],["0","1","1","1","1","1","1","1","0","1","1","1","1","1","1","1","1","1","1","1"],["1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1"],["1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1"],["1","1","1","1","1","0","1","1","1","1","1","1","1","0","1","1","1","1","1","1"],["1","0","1","1","1","1","1","0","1","1","1","0","1","1","1","1","0","1","1","1"],["1","1","1","1","1","1","1","1","1","1","1","1","0","1","1","1","1","1","1","0"],["1","1","1","1","1","1","1","1","1","1","1","1","1","0","1","1","1","1","0","0"],["1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1"],["1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1"],["1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1","1"]]
+    
+    s = Solution()
+    starttime = int(time())
+    print(f"Number of islands: {s.numIslands(grid2)}")
+    endtime = int(time())
+    totalTime = endtime - starttime
+    print(f"Total time was {totalTime}")
+