Merge pull request #172 from sidaksohi/add-search-solutions-with-descriptions

Add number_of_islands to 'searches', with description

searches/number_of_islands.rb

@@ -0,0 +1,79 @@
+#Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands.
+#An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.
+
+#Example 1:
+#Input: grid = [
+#  ["1","1","1","1","0"],
+#  ["1","1","0","1","0"],
+#  ["1","1","0","0","0"],
+#  ["0","0","0","0","0"]
+#]
+#Output: 1
+
+#Example 2:
+#Input: grid = [
+#  ["1","1","0","0","0"],
+#  ["1","1","0","0","0"],
+#  ["0","0","1","0","0"],
+#  ["0","0","0","1","1"]
+#]
+#Output: 3
+
+#Constraints:
+#m == grid.length
+#n == grid[i].length
+#1 <= m, n <= 300
+#grid[i][j] is '0' or '1'.
+
+
+
+
+#DFS, Recursive Bottom Up Approach - O(n*m) Time / O(1) Space
+#Init num_of_islands = 0, return if the grid is empty
+#Start a double loop with index to iterate through each plot (each value is a plot of either water or land in this case)
+#if the plot is land, dfs(grid, x, y)
+#num_of_islands += 1
+#Return num_of_islands
+
+#dfs(grid, x, y)
+#Return if x or y are out of bounds, or if the plot is water
+#Make the current plot water
+#Call dfs again for up, down, left, and right     
+
+# @param {Character[][]} grid
+# @return {Integer}
+def num_islands(grid)
+    return 0 if grid.empty?
+    
+    #init num of islands
+    islands = 0
+    
+    #loop through each element (plot) in the 2d array
+    grid.each_with_index do |row, x|
+      row.each_with_index do |plot, y|
+        #if the plot is water, start a dfs
+        if plot == "1"
+          dfs(grid, x, y)
+          #add 1 to islands once all connected land plots are searched
+          islands += 1
+        end
+      end
+    end
+    
+    #return ans
+    islands
+end
+
+def dfs(grid, x, y)
+    #don't search if out of bounds, or if it's already water
+    return if x < 0 || x >= grid.length || y < 0 || y >= grid[0].length || grid[x][y] == "0"
+    
+    #set the plot to water
+    grid[x][y] = "0"
+    
+    #search each adjacent plot
+    dfs(grid, x - 1, y) #up
+    dfs(grid, x + 1, y) #down
+    dfs(grid, x, y - 1) #left
+    dfs(grid, x, y + 1) #right
+end