diff --git a/Gemfile.lock b/Gemfile.lock
new file mode 100644
index 0000000..cae865a
--- /dev/null
+++ b/Gemfile.lock
@@ -0,0 +1,30 @@
+GEM
+  remote: http://rubygems.org/
+  specs:
+    ansi (1.5.0)
+    builder (3.2.3)
+    minitest (5.10.3)
+    minitest-reporters (1.3.8)
+      ansi
+      builder
+      minitest (>= 5.0)
+      ruby-progressbar
+    minitest-skip (0.0.1)
+      minitest (~> 5.0)
+    rake (12.3.0)
+    ruby-progressbar (1.10.1)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  minitest
+  minitest-reporters
+  minitest-skip
+  rake
+
+RUBY VERSION
+   ruby 2.5.1p57
+
+BUNDLED WITH
+   1.17.3
diff --git a/lib/reverse_sentence.rb b/lib/reverse_sentence.rb
index 3fe7cac..0b61cc7 100644
--- a/lib/reverse_sentence.rb
+++ b/lib/reverse_sentence.rb
@@ -1,6 +1,39 @@
 # A method to reverse the words in a sentence, in place.
-# Time complexity: ?
-# Space complexity: ?
-def reverse_sentence(my_sentence)
-  raise NotImplementedError
+# Time complexity: O(n*m)???
+# Space complexity: O(1)
+def reverse_helper(string, s, e)
+  pairs = (e + 1 - s)/2
+  pairs.times do
+    temp = string[s]
+    string[s] = string[e]
+    string[e] = temp
+    s += 1
+    e -= 1
+  end
 end
+
+def reverse_sentence(sentence) 
+  return sentence if sentence == nil || sentence == "" || sentence.include?("\s") == false
+  reverse_helper(sentence, 0, (sentence.length - 1))
+
+  j = 0
+  spaces = []
+  while j <= sentence.length
+    if sentence[j] == " "
+      spaces << j
+      if spaces.length == 1
+        # reverse the first word
+        reverse_helper(sentence, 0, (j - 1))
+      elsif spaces.length > 1
+        # reverse the middle word
+        reverse_helper(sentence, (spaces[-2] + 1), (j - 1))
+      end
+    elsif sentence[j] == nil
+      # reverse the last word
+      reverse_helper(sentence, (spaces[-1] + 1), (j - 1))
+    end
+    j += 1
+  end
+
+  return sentence
+end
\ No newline at end of file
diff --git a/lib/sort_by_length.rb b/lib/sort_by_length.rb
index a5713ad..ee7a722 100644
--- a/lib/sort_by_length.rb
+++ b/lib/sort_by_length.rb
@@ -1,7 +1,23 @@
 # A method which will return an array of the words in the string
-#  sorted by the length of the word.
-# Time complexity: ?
-# Space complexity: ?
-def sort_by_length(my_sentence)
-  raise NotImplementedError, "Method not implemented"
-end
+# sorted by the length of the word.
+# Time complexity: O(n2)
+# Space complexity: O(n)
+
+def sort_by_length(sentence)
+  #Step_1: break the sentence into an array of words
+  array = sentence.split(/\W+/)
+  #Step_2: use insertion sort to sort the array
+  i = 1
+  while i < array.length
+    to_insert = array[i]
+    j = i
+    while j > 0 && array[j-1].length > to_insert.length
+      array[j] = array[j-1]
+      j -= 1
+    end
+    array[j] = to_insert
+    i += 1
+  end
+
+  return array
+end
\ No newline at end of file
diff --git a/test/sort_by_length.rb b/test/sort_by_length_test.rb
similarity index 100%
rename from test/sort_by_length.rb
rename to test/sort_by_length_test.rb