diff --git a/arrays_strings/permutation/permutation_challenge.ipynb b/arrays_strings/permutation/permutation_challenge.ipynb
index 221b0268..4d9c61fc 100644
--- a/arrays_strings/permutation/permutation_challenge.ipynb
+++ b/arrays_strings/permutation/permutation_challenge.ipynb
@@ -124,6 +124,7 @@
     "    def test_permutation(self, func):\n",
     "        assert_equal(func(None, 'foo'), False)\n",
     "        assert_equal(func('', 'foo'), False)\n",
+    "        assert_equal(func('', ''), False)\n",
     "        assert_equal(func('Nib', 'bin'), False)\n",
     "        assert_equal(func('act', 'cat'), True)\n",
     "        assert_equal(func('a ct', 'ca t'), True)\n",
@@ -159,7 +160,7 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3",
+   "display_name": "Python [default]",
    "language": "python",
    "name": "python3"
   },
@@ -173,9 +174,9 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.5.0"
+   "version": "3.6.3"
   }
  },
  "nbformat": 4,
- "nbformat_minor": 0
+ "nbformat_minor": 2
 }
diff --git a/arrays_strings/permutation/permutation_solution.ipynb b/arrays_strings/permutation/permutation_solution.ipynb
index 2cca0597..46e32aff 100644
--- a/arrays_strings/permutation/permutation_solution.ipynb
+++ b/arrays_strings/permutation/permutation_solution.ipynb
@@ -90,7 +90,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 1,
+   "execution_count": 21,
    "metadata": {
     "collapsed": false
    },
@@ -99,7 +99,7 @@
     "class Permutations(object):\n",
     "\n",
     "    def is_permutation(self, str1, str2):\n",
-    "        if str1 is None or str2 is None:\n",
+    "        if not str1 or not str2:\n",
     "            return False\n",
     "        return sorted(str1) == sorted(str2)"
    ]
@@ -141,7 +141,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 2,
+   "execution_count": 24,
    "metadata": {
     "collapsed": false
    },
@@ -153,7 +153,7 @@
     "class PermutationsAlt(object):\n",
     "\n",
     "    def is_permutation(self, str1, str2):\n",
-    "        if str1 is None or str2 is None:\n",
+    "        if not str1 or not str2:\n",
     "            return False\n",
     "        if len(str1) != len(str2):\n",
     "            return False\n",
@@ -166,6 +166,55 @@
     "        return unique_counts1 == unique_counts2"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm: Array Removal\n",
+    " \n",
+    "Since a permuation is a symmetric relation (A is a permutation of B iff B is a permutation of A) one only needs to check if one can rebuild the first string using all the characters of the second.\n",
+    "\n",
+    "Steps:\n",
+    "* Check if the strings are not None or empty\n",
+    "* Turn the second string into a list of characters\n",
+    "* For each character in the first string:\n",
+    "    * If the character does not appear in the list, return False\n",
+    "    * Else: remove the character from the list\n",
+    "* Return True if the list is empty (we used up all characters), else return False\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: O(n)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Code: Array Lookup"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 39,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "class PermutationsArray(object):\n",
+    "\n",
+    "    def is_permutation(self, str1, str2):\n",
+    "        if not str1 or not str2:\n",
+    "            return False\n",
+    "        \n",
+    "        chars = list(str2)       \n",
+    "        for char in str1:\n",
+    "            if char not in chars:\n",
+    "                return False\n",
+    "            else:\n",
+    "                chars.remove(char)\n",
+    "        return len(chars) == 0"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -175,7 +224,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 3,
+   "execution_count": 40,
    "metadata": {
     "collapsed": false
    },
@@ -198,6 +247,7 @@
     "    def test_permutation(self, func):\n",
     "        assert_equal(func(None, 'foo'), False)\n",
     "        assert_equal(func('', 'foo'), False)\n",
+    "        assert_equal(func('', ''), False)\n",
     "        assert_equal(func('Nib', 'bin'), False)\n",
     "        assert_equal(func('act', 'cat'), True)\n",
     "        assert_equal(func('a ct', 'ca t'), True)\n",
@@ -212,6 +262,8 @@
     "    try:\n",
     "        permutations_alt = PermutationsAlt()\n",
     "        test.test_permutation(permutations_alt.is_permutation)\n",
+    "        permutations_array = PermutationsArray()\n",
+    "        test.test_permutation(permutations_array.is_permutation)\n",
     "    except NameError:\n",
     "        # Alternate solutions are only defined\n",
     "        # in the solutions file\n",
@@ -224,7 +276,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 4,
+   "execution_count": 41,
    "metadata": {
     "collapsed": false
    },
@@ -233,6 +285,7 @@
      "name": "stdout",
      "output_type": "stream",
      "text": [
+      "Success: test_permutation\n",
       "Success: test_permutation\n",
       "Success: test_permutation\n"
      ]
@@ -245,7 +298,7 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3",
+   "display_name": "Python [default]",
    "language": "python",
    "name": "python3"
   },
@@ -259,9 +312,9 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.5.0"
+   "version": "3.6.3"
   }
  },
  "nbformat": 4,
- "nbformat_minor": 0
+ "nbformat_minor": 2
 }
diff --git a/arrays_strings/permutation/test_permutation_solution.py b/arrays_strings/permutation/test_permutation_solution.py
index 9ab124c4..8af92b32 100644
--- a/arrays_strings/permutation/test_permutation_solution.py
+++ b/arrays_strings/permutation/test_permutation_solution.py
@@ -6,6 +6,7 @@ class TestPermutation(object):
     def test_permutation(self, func):
         assert_equal(func(None, 'foo'), False)
         assert_equal(func('', 'foo'), False)
+        assert_equal(func('', ''), False)
         assert_equal(func('Nib', 'bin'), False)
         assert_equal(func('act', 'cat'), True)
         assert_equal(func('a ct', 'ca t'), True)
@@ -20,6 +21,8 @@ def main():
     try:
         permutations_alt = PermutationsAlt()
         test.test_permutation(permutations_alt.is_permutation)
+        permutations_array = PermutationsArray()
+        test.test_permutation(permutations_array.is_permutation)
     except NameError:
         # Alternate solutions are only defined
         # in the solutions file