implement max_sub_array

Author: geneminde

lib/max_subarray.rb

@@ -1,8 +1,18 @@
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n)
+# Space Complexity: O(1)
 def max_sub_array(nums)
-    return 0 if nums == nil
-    
-    raise NotImplementedError, "Method not implemented yet!"
+    return nil if nums.empty?
+
+    max_so_far = nums[0]
+    max_ending_here = nums[0]
+
+    nums.each_with_index do |number, index|
+        if index > 0
+            max_ending_here = [number, max_ending_here + number].max
+            max_so_far = [max_so_far, max_ending_here].max
+        end
+    end
+
+    return max_so_far
 end

test/max_sub_array_test.rb

@@ -1,6 +1,6 @@
 require_relative "test_helper"
 
-xdescribe "max subarray" do
+describe "max subarray" do
   it "will work for [-2,1,-3,4,-1,2,1,-5,4]" do
     # Arrange
     input = [-2,1,-3,4,-1,2,1,-5,4]
@@ -46,14 +46,14 @@
   end
 
   it "will return nil for an empty array" do
-   # Arrange
-   input = []
+    # Arrange
+    input = []
 
-   # Act
-   answer = max_sub_array(input)
+    # Act
+    answer = max_sub_array(input)
 
-   # Assert
-   expect(answer).must_be_nil
+    # Assert
+    expect(answer).must_be_nil
   end
 
   it "will work for [50, -50, 50]" do