diff --git a/lib/practice_exercises.rb b/lib/practice_exercises.rb
index 291e4e6..bdfefc0 100644
--- a/lib/practice_exercises.rb
+++ b/lib/practice_exercises.rb
@@ -1,13 +1,60 @@
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n)
+# Space Complexity: O(1)
 def remove_duplicates(list)
-  raise NotImplementedError, "Not implemented yet"
+  raise ArgumentError if !list
+  length = list.length
+  
+  if length > 0
+    unique_index = 0
+    unique_element = list.first
+    
+    length.times do |index|
+      if list[index] != unique_element
+        unique_element = list[index]
+        unique_index += 1
+        list[unique_index] = unique_element
+      end
+      
+      if index > unique_index
+        list[index] = nil
+      end
+    end
+    
+  end
+  return list
 end
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n * m) where n is the number of strings in input array, m is the length
+# (number of characters) of either prefix string or current string. 
+# Space Complexity: O(1) because this method uses constant number of temp variables and returns 
+# only 1 string (it might contain m characters but still only 1 string)
+
 def longest_prefix(strings)
-  raise NotImplementedError, "Not implemented yet"
+  raise ArgumentError if !strings
+  
+  prefix = ""
+  if strings.length > 0
+    prefix = strings[0]
+    (strings.length - 1).times do |i|
+      current_string = strings[i + 1]
+
+      if prefix.length < current_string.length 
+        length = prefix.length
+      else
+        length = current_string.length
+        prefix = prefix[0...length]
+      end
+
+      length.times do |j|
+        if prefix[j] != current_string[j]
+          prefix = prefix[0...j]
+          break
+        end
+      end
+    end
+  end
+
+  return prefix
 end
 
diff --git a/test/practice_exercises_test.rb b/test/practice_exercises_test.rb
index 4d0bc10..66927cb 100644
--- a/test/practice_exercises_test.rb
+++ b/test/practice_exercises_test.rb
@@ -39,5 +39,13 @@
 
       expect(output).must_equal ""
     end
+
+    it "will update shorter prefix strings" do
+      strings = ["bbb","bb"]
+
+      output = longest_prefix(strings)
+
+      expect(output).must_equal "bb"
+    end
   end
 end