Merge pull request #44 from donnemartin/develop

Polish compress string solution

arrays_strings/compress/compress_solution.ipynb

@@ -18,7 +18,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Problem: Compress a string such that 'AAABCCDDDD' becomes 'A3B1C2D4'.  Only compress the string if it saves space.\n",
+    "## Problem: Compress a string such that 'AAABCCDDDD' becomes 'A3BC2D4'.  Only compress the string if it saves space.\n",
     "\n",
     "* [Constraints](#Constraints)\n",
     "* [Test Cases](#Test-Cases)\n",
@@ -51,7 +51,7 @@
     "* None -> None\n",
     "* '' -> ''\n",
     "* 'AABBCC' -> 'AABBCC'\n",
-    "* 'AAABCCDDDD' -> 'A3B1C2D4'"
+    "* 'AAABCCDDDD' -> 'A3BC2D4'"
    ]
   },
   {
@@ -60,26 +60,24 @@
    "source": [
     "## Algorithm\n",
     "\n",
-    "Since Python strings are immutable, we'll use a list of characters to build the compressed string representation.  We'll then convert the list to a string.\n",
-    "\n",
-    "* Calculate the size of the compressed string\n",
-    "    * Note the constraint about compressing only if it saves space\n",
-    "* If the compressed string size is >= string size, return string\n",
-    "* Create compressed_string\n",
-    "    * For each char in string\n",
-    "        * If char is the same as last_char, increment count\n",
-    "        * Else\n",
-    "            * Append last_char to compressed_string\n",
-    "            * append count to compressed_string\n",
-    "            * count = 1\n",
-    "            * last_char = char\n",
-    "        * Append last_char to compressed_string\n",
-    "        * Append count to compressed_string\n",
-    "    * Return compressed_string\n",
+    "* For each char in string\n",
+    "    * If char is the same as last_char, increment count\n",
+    "    * Else\n",
+    "        * Append last_char and count to compressed_string\n",
+    "        * last_char = char\n",
+    "        * count = 1\n",
+    "* Append last_char and count to compressed_string\n",
+    "* If the compressed string size is < string size\n",
+    "    * Return compressed string\n",
+    "* Else\n",
+    "    * Return string\n",
     "\n",
     "Complexity:\n",
     "* Time: O(n)\n",
-    "* Space: O(n)"
+    "* Space: O(n)\n",
+    "\n",
+    "Complexity Note:\n",
+    "* Although strings are immutable in Python, appending to strings is optimized in CPython so that it now runs in O(n) and extends the string in-place.  Refer to this [Stack Overflow post](http://stackoverflow.com/a/4435752)."
    ]
   },
   {
@@ -100,38 +98,18 @@
     "def compress_string(string):\n",
     "    if string is None or len(string) == 0:\n",
     "        return string\n",
-    "\n",
-    "    # Calculate the size of the compressed string\n",
-    "    size = 0\n",
-    "    last_char = string[0]\n",
-    "    for char in string:\n",
-    "        if char != last_char:\n",
-    "            size += 2\n",
-    "            last_char = char\n",
-    "    size += 2\n",
-    "\n",
-    "    # If the compressed string size is greater than\n",
-    "    # or equal to string size, return original string\n",
-    "    if size >= len(string):\n",
-    "        return string\n",
-    "\n",
-    "    # Create compressed_string\n",
-    "    compressed_string = list()\n",
+    "    result = ''\n",
+    "    prev_char = string[0]\n",
     "    count = 0\n",
-    "    last_char = string[0]\n",
     "    for char in string:\n",
-    "        if char == last_char:\n",
+    "        if char == prev_char:\n",
     "            count += 1\n",
     "        else:\n",
-    "            compressed_string.append(last_char)\n",
-    "            compressed_string.append(str(count))\n",
+    "            result += prev_char + (str(count) if count > 1 else '')\n",
+    "            prev_char = char\n",
     "            count = 1\n",
-    "            last_char = char\n",
-    "    compressed_string.append(last_char)\n",
-    "    compressed_string.append(str(count))\n",
-    "\n",
-    "    # Convert the characters in the list to a string\n",
-    "    return \"\".join(compressed_string)"
+    "    result += prev_char + str(count)\n",
+    "    return result if len(result) < len(string) else string"
    ]
   },
   {
@@ -167,7 +145,7 @@
     "        assert_equal(func(None), None)\n",
     "        assert_equal(func(''), '')\n",
     "        assert_equal(func('AABBCC'), 'AABBCC')\n",
-    "        assert_equal(func('AAABCCDDDD'), 'A3B1C2D4')\n",
+    "        assert_equal(func('AAABCCDDDD'), 'A3BC2D4')\n",
     "        print('Success: test_compress')\n",
     "\n",
     "\n",
@@ -202,21 +180,21 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 2",
+   "display_name": "Python 3",
    "language": "python",
-   "name": "python2"
+   "name": "python3"
   },
   "language_info": {
    "codemirror_mode": {
     "name": "ipython",
-    "version": 2
+    "version": 3
    },
    "file_extension": ".py",
    "mimetype": "text/x-python",
    "name": "python",
    "nbconvert_exporter": "python",
-   "pygments_lexer": "ipython2",
-   "version": "2.7.10"
+   "pygments_lexer": "ipython3",
+   "version": "3.4.3"
   }
  },
  "nbformat": 4,

arrays_strings/compress/test_compress.py

@@ -7,7 +7,7 @@ class TestCompress(object):
         assert_equal(func(None), None)
         assert_equal(func(''), '')
         assert_equal(func('AABBCC'), 'AABBCC')
-        assert_equal(func('AAABCCDDDD'), 'A3B1C2D4')
+        assert_equal(func('AAABCCDDDD'), 'A3BC2D4')
         print('Success: test_compress')