Add assumption to str diff challenge to match the suggested solution (#178)

arrays_strings/str_diff/str_diff_challenge.ipynb

@@ -34,6 +34,7 @@
    "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",

arrays_strings/str_diff/str_diff_solution.ipynb

@@ -76,7 +76,7 @@
     "\n",
     "Complexity:\n",
     "* Time: O(m+n), where m and n are the lengths of s, t\n",
-    "* Space: O(1), for the dict, where h is the unique chars in s"
+    "* Space: O(1)"
    ]
   },
   {