diff --git a/lib/practice_exercises.rb b/lib/practice_exercises.rb
index 291e4e6..a8d6ff9 100644
--- a/lib/practice_exercises.rb
+++ b/lib/practice_exercises.rb
@@ -1,13 +1,49 @@
+require "pry"
+# Time Complexity: o(n)
+# Space Complexity: o(1)
 
-# Time Complexity: ?
-# Space Complexity: ?
+# num = [1, 2, 2, 3, 4, 4]
 def remove_duplicates(list)
-  raise NotImplementedError, "Not implemented yet"
+  i = 0
+  while i <= list.length
+    if list[i] == list[i + 1]
+      list.delete_at(i)
+    end
+    i += 1
+  end
+
+  # print list.length
+  return list
 end
 
-# Time Complexity: ?
-# Space Complexity: ?
+# remove_duplicates(num)
+#==============================================================
+
+# Time Complexity: o(n^2)
+# Space Complexity: ?o(n)
+
 def longest_prefix(strings)
-  raise NotImplementedError, "Not implemented yet"
-end
+  total_words = strings.length
+  prefix = ""
 
+  if total_words == 0
+    return prefix
+  end
+
+  if total_words == 1
+    return strings[0]
+  end
+
+  total_words.times do |index|
+    word = strings[index]
+    i = index + 1
+    while i < total_words
+      next_word = strings[i]
+      if prefix[index] != word[index] && word[index] == next_word[index]
+        prefix += word[index]
+      end
+      i += 1
+    end
+  end
+  return prefix
+end
diff --git a/test/practice_exercises_test.rb b/test/practice_exercises_test.rb
index 4d0bc10..4a9108b 100644
--- a/test/practice_exercises_test.rb
+++ b/test/practice_exercises_test.rb
@@ -5,19 +5,19 @@
     it "works for 1 element strings" do
       expect(remove_duplicates([1])).must_equal [1]
     end
-    
+
     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]).reject{|num| num == nil }).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"]
+      strings = ["flower", "flow", "flight"]
 
       output = longest_prefix(strings)
 
@@ -25,7 +25,7 @@
     end
 
     it "will work for the strings with the common prefix in the rear" do
-      strings = ["flower","flow","flight", "fpastafl"]
+      strings = ["flower", "flow", "flight", "fpastafl"]
 
       output = longest_prefix(strings)
 
@@ -33,7 +33,7 @@
     end
 
     it "will work for the README strings" do
-      strings = ["dog","racecar","car"]
+      strings = ["dog", "racecar", "car"]
 
       output = longest_prefix(strings)