diff --git a/arrays_strings/str_diff/str_diff_challenge.ipynb b/arrays_strings/str_diff/str_diff_challenge.ipynb
index b4d0ff0..75f9291 100644
--- a/arrays_strings/str_diff/str_diff_challenge.ipynb
+++ b/arrays_strings/str_diff/str_diff_challenge.ipynb
@@ -34,13 +34,13 @@
    "source": [
     "## Constraints\n",
     "\n",
-    "* Assume two strings str1, str2, where str2 contains the same set of characters in str1 with one additional character.\n",
     "* Can we assume the strings are ASCII?\n",
     "    * Yes\n",
     "* Is case important?\n",
     "    * The strings are lower case\n",
     "* Can we assume the inputs are valid?\n",
     "    * No, check for None\n",
+    "    * Otherwise, assume there is only a single different char between the two strings\n",
     "* Can we assume this fits memory?\n",
     "    * Yes"
    ]
@@ -52,6 +52,8 @@
     "## Test Cases\n",
     "\n",
     "* None input -> TypeError\n",
+    "* 'ab', 'aab' -> 'a'\n",
+    "* 'aab', 'ab' -> 'a'\n",
     "* 'abcd', 'abcde' -> 'e'\n",
     "* 'aaabbcdd', 'abdbacade' -> 'e'"
    ]
@@ -82,7 +84,7 @@
    "source": [
     "class Solution(object):\n",
     "\n",
-    "    def find_diff(self, s, t):\n",
+    "    def find_diff(self, str1, str2):\n",
     "        # TODO: Implement me\n",
     "        pass"
    ]
@@ -117,9 +119,15 @@
     "\n",
     "    def test_find_diff(self):\n",
     "        solution = Solution()\n",
-    "        assert_raises(TypeError, solution.find_diff, None, None)\n",
+    "        assert_raises(TypeError, solution.find_diff, None)\n",
+    "        assert_equal(solution.find_diff('ab', 'aab'), 'a')\n",
+    "        assert_equal(solution.find_diff('aab', 'ab'), 'a')\n",
     "        assert_equal(solution.find_diff('abcd', 'abcde'), 'e')\n",
     "        assert_equal(solution.find_diff('aaabbcdd', 'abdbacade'), 'e')\n",
+    "        assert_equal(solution.find_diff_xor('ab', 'aab'), 'a')\n",
+    "        assert_equal(solution.find_diff_xor('aab', 'ab'), 'a')\n",
+    "        assert_equal(solution.find_diff_xor('abcd', 'abcde'), 'e')\n",
+    "        assert_equal(solution.find_diff_xor('aaabbcdd', 'abdbacade'), 'e')\n",
     "        print('Success: test_find_diff')\n",
     "\n",
     "\n",
diff --git a/arrays_strings/str_diff/str_diff_solution.ipynb b/arrays_strings/str_diff/str_diff_solution.ipynb
index 23b1471..9277e25 100644
--- a/arrays_strings/str_diff/str_diff_solution.ipynb
+++ b/arrays_strings/str_diff/str_diff_solution.ipynb
@@ -39,6 +39,7 @@
     "    * The strings are lower case\n",
     "* Can we assume the inputs are valid?\n",
     "    * No, check for None\n",
+    "    * Otherwise, assume there is only a single different char between the two strings\n",
     "* Can we assume this fits memory?\n",
     "    * Yes"
    ]
@@ -50,6 +51,8 @@
     "## Test Cases\n",
     "\n",
     "* None input -> TypeError\n",
+    "* 'ab', 'aab' -> 'a'\n",
+    "* 'aab', 'ab' -> 'a'\n",
     "* 'abcd', 'abcde' -> 'e'\n",
     "* 'aaabbcdd', 'abdbacade' -> 'e'"
    ]
@@ -65,6 +68,7 @@
     "* Keep a dictionary of seen values in s\n",
     "* Loop through t, decrementing the seen values\n",
     "    * If the char is not there or if the decrement results in a negative value, return the char\n",
+    "* Return the differing char from the dictionary\n",
     "\n",
     "Complexity:\n",
     "* Time: O(m+n), where m and n are the lengths of s, t\n",
@@ -112,7 +116,8 @@
     "                return char\n",
     "            if seen[char] < 0:\n",
     "                return char\n",
-    "        return None\n",
+    "        for char, count in seen.items():\n",
+    "            return char\n",
     "\n",
     "    def find_diff_xor(self, str1, str2):\n",
     "        if str1 is None or str2 is None:\n",
@@ -157,8 +162,14 @@
     "    def test_find_diff(self):\n",
     "        solution = Solution()\n",
     "        assert_raises(TypeError, solution.find_diff, None)\n",
+    "        assert_equal(solution.find_diff('ab', 'aab'), 'a')\n",
+    "        assert_equal(solution.find_diff('aab', 'ab'), 'a')\n",
     "        assert_equal(solution.find_diff('abcd', 'abcde'), 'e')\n",
     "        assert_equal(solution.find_diff('aaabbcdd', 'abdbacade'), 'e')\n",
+    "        assert_equal(solution.find_diff_xor('ab', 'aab'), 'a')\n",
+    "        assert_equal(solution.find_diff_xor('aab', 'ab'), 'a')\n",
+    "        assert_equal(solution.find_diff_xor('abcd', 'abcde'), 'e')\n",
+    "        assert_equal(solution.find_diff_xor('aaabbcdd', 'abdbacade'), 'e')\n",
     "        print('Success: test_find_diff')\n",
     "\n",
     "\n",
diff --git a/arrays_strings/str_diff/test_str_diff.py b/arrays_strings/str_diff/test_str_diff.py
index 7807db3..ca9bd26 100644
--- a/arrays_strings/str_diff/test_str_diff.py
+++ b/arrays_strings/str_diff/test_str_diff.py
@@ -6,8 +6,14 @@ class TestFindDiff(object):
     def test_find_diff(self):
         solution = Solution()
         assert_raises(TypeError, solution.find_diff, None)
+        assert_equal(solution.find_diff('ab', 'aab'), 'a')
+        assert_equal(solution.find_diff('aab', 'ab'), 'a')
         assert_equal(solution.find_diff('abcd', 'abcde'), 'e')
         assert_equal(solution.find_diff('aaabbcdd', 'abdbacade'), 'e')
+        assert_equal(solution.find_diff_xor('ab', 'aab'), 'a')
+        assert_equal(solution.find_diff_xor('aab', 'ab'), 'a')
+        assert_equal(solution.find_diff_xor('abcd', 'abcde'), 'e')
+        assert_equal(solution.find_diff_xor('aaabbcdd', 'abdbacade'), 'e')
         print('Success: test_find_diff')