Add functional solution to compress string challenge (#103)

arrays_strings/compress_alt/better_compress_solution.ipynb

@@ -4,7 +4,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "This notebook was prepared by [hashhar](https://github.com/hashhar). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
+    "This notebook was prepared by [hashhar](https://github.com/hashhar), second solution added by [janhak] (https://github.com/janhak). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
    ]
   },
   {
@@ -103,7 +103,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 1,
+   "execution_count": null,
    "metadata": {
     "collapsed": false
    },
@@ -164,6 +164,72 @@
     "    return \"\".join(compressed_string)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Algorithm: Split to blocks and compress\n",
+    "\n",
+    "Let us split the string first into blocks of identical characters and then compress it block by block.\n",
+    "\n",
+    "* Split the string to blocks\n",
+    "    * For each character in string\n",
+    "        * Add this character to block\n",
+    "        * If the next character is different\n",
+    "            * Return block\n",
+    "            * Erase the content of block\n",
+    "\n",
+    "\n",
+    "* Compress block\n",
+    "    *  If block consists of two or fewer characters\n",
+    "        * Return block\n",
+    "    * Else\n",
+    "        * Append length of the block to the first character and return\n",
+    "\n",
+    "\n",
+    "* Compress string\n",
+    "    * Split the string to blocks\n",
+    "    * Compress blocks\n",
+    "    * Join compressed blocks\n",
+    "    * Return result if it is shorter than original string\n",
+    "\n",
+    "Complexity:\n",
+    "* Time: O(n)\n",
+    "* Space: O(n)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "def split_to_blocks(string):\n",
+    "    block = ''\n",
+    "    for char, next_char in zip(string, string[1:] + ' '):\n",
+    "        block += char\n",
+    "        if char is not next_char:\n",
+    "            yield block\n",
+    "            block = ''\n",
+    "\n",
+    "\n",
+    "def compress_block(block):\n",
+    "    if len(block) <= 2:\n",
+    "        return block\n",
+    "    else:\n",
+    "        return block[0] + str(len(block))\n",
+    "\n",
+    "\n",
+    "def compress_string(string):\n",
+    "    if string is None or not string:\n",
+    "        return string\n",
+    "    compressed = (compress_block(block) for block in split_to_blocks(string))\n",
+    "    result = ''.join(compressed)\n",
+    "    return result if len(result) < len(string) else string"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -173,19 +239,11 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 2,
+   "execution_count": null,
    "metadata": {
     "collapsed": false
    },
-   "outputs": [
-    {
-     "name": "stdout",
-     "output_type": "stream",
-     "text": [
-      "Overwriting test_compress.py\n"
-     ]
-    }
-   ],
+   "outputs": [],
    "source": [
     "%%writefile test_compress.py\n",
     "from nose.tools import assert_equal\n",
@@ -213,19 +271,11 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 3,
+   "execution_count": null,
    "metadata": {
     "collapsed": false
    },
-   "outputs": [
-    {
-     "name": "stdout",
-     "output_type": "stream",
-     "text": [
-      "Success: test_compress\n"
-     ]
-    }
-   ],
+   "outputs": [],
    "source": [
     "%run -i test_compress.py"
    ]