diff --git a/lib/practice_exercises.rb b/lib/practice_exercises.rb
index 291e4e6..cf71eda 100644
--- a/lib/practice_exercises.rb
+++ b/lib/practice_exercises.rb
@@ -1,13 +1,45 @@
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(n)
+# Space Complexity: O(1)
 def remove_duplicates(list)
-  raise NotImplementedError, "Not implemented yet"
+  index = 0
+  prev_num = 0
+  list.length.times do
+    item = list[index]
+    if index == 0
+      prev_num = item
+      index += 1
+    else
+      if item == prev_num
+        list.delete_at(index)
+      else
+        index += 1
+        prev_num = item
+      end
+    end
+  end
+  return list
 end
 
-# Time Complexity: ?
-# Space Complexity: ?
+# Time Complexity: O(a * b) where a is the number of letters in the first word, and b is the number of words in the array
+# Space Complexity: O(1)
 def longest_prefix(strings)
-  raise NotImplementedError, "Not implemented yet"
+  longest_prefix = ""
+  num_letters = strings[0].length
+  
+  num_letters.times do |i|
+    status = true
+    check_letter = strings[0][i]
+    strings.each do |string|
+      if string[i] != check_letter
+        status = false
+      end
+    end
+    if status == true
+      longest_prefix += check_letter
+    end
+  end
+  
+  return longest_prefix
 end
 
diff --git a/test/practice_exercises_test.rb b/test/practice_exercises_test.rb
index 11d820b..cb96011 100644
--- a/test/practice_exercises_test.rb
+++ b/test/practice_exercises_test.rb
@@ -9,35 +9,51 @@
     it "works for empty arrays" do
       expect(remove_duplicates([])).must_equal []
     end
-
+    
     it "will remove duplicates for longer arrays" do
-      expect(remove_duplicates([1, 2, 2, 3, 3, 4])).must_equal [1, 2, 3, 4]
+      expect(remove_duplicates([1, 2, 2, 3, 3, 4]).reject{|num| num == nil }).must_equal [1, 2, 3, 4]
     end
   end
-
+  
   describe "Longest valid substring" do
     it "will work for the README strings" do
       strings = ["flower","flow","flight"]
-
+      
       output = longest_prefix(strings)
-
+      
       expect(output).must_equal "fl"
     end
-
+    
     it "will work for the strings with the common prefix in the rear" do
       strings = ["flower","flow","flight", "fpastafl"]
-
+      
       output = longest_prefix(strings)
-
+      
       expect(output).must_equal "f"
     end
-
+    
     it "will work for the README strings" do
       strings = ["dog","racecar","car"]
-
+      
       output = longest_prefix(strings)
-
+      
       expect(output).must_equal ""
     end
+    
+    it "will work when the first word is the longest" do
+      strings = ["racecar","race","racer"]
+      
+      output = longest_prefix(strings)
+      
+      expect(output).must_equal "race"
+    end
+    
+    it "will work with words that are the same" do
+      strings = ["flower","flower","flower"]
+      
+      output = longest_prefix(strings)
+      
+      expect(output).must_equal "flower"
+    end
   end
 end