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Merge pull request #44 from donnemartin/develop

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Polish compress string solution
This commit is contained in:
Donne Martin 2016-02-09 06:54:46 -05:00
commit b53d7ba9ef
2 changed files with 31 additions and 53 deletions
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82
arrays_strings/compress/compress_solution.ipynb
View File

@ -18,7 +18,7 @@
"cell_type": "markdown", "cell_type": "markdown",
"metadata": {}, "metadata": {},
"source": [ "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", "\n",
"* [Constraints](#Constraints)\n", "* [Constraints](#Constraints)\n",
"* [Test Cases](#Test-Cases)\n", "* [Test Cases](#Test-Cases)\n",
@ -51,7 +51,7 @@
"* None -> None\n", "* None -> None\n",
"* '' -> ''\n", "* '' -> ''\n",
"* 'AABBCC' -> 'AABBCC'\n", "* 'AABBCC' -> 'AABBCC'\n",
"* 'AAABCCDDDD' -> 'A3B1C2D4'" "* 'AAABCCDDDD' -> 'A3BC2D4'"
] ]
}, },
{ {
@ -60,26 +60,24 @@
"source": [ "source": [
"## Algorithm\n", "## Algorithm\n",
"\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", "* For each char in string\n",
"\n", " * If char is the same as last_char, increment count\n",
"* Calculate the size of the compressed string\n", " * Else\n",
" * Note the constraint about compressing only if it saves space\n", " * Append last_char and count to compressed_string\n",
"* If the compressed string size is >= string size, return string\n", " * last_char = char\n",
"* Create compressed_string\n", " * count = 1\n",
" * For each char in string\n", "* Append last_char and count to compressed_string\n",
" * If char is the same as last_char, increment count\n", "* If the compressed string size is < string size\n",
" * Else\n", " * Return compressed string\n",
" * Append last_char to compressed_string\n", "* Else\n",
" * append count to compressed_string\n", " * Return 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",
"\n", "\n",
"Complexity:\n", "Complexity:\n",
"* Time: O(n)\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", "def compress_string(string):\n",
" if string is None or len(string) == 0:\n", " if string is None or len(string) == 0:\n",
" return string\n", " return string\n",
"\n", " result = ''\n",
" # Calculate the size of the compressed string\n", " prev_char = string[0]\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",
" count = 0\n", " count = 0\n",
" last_char = string[0]\n",
" for char in string:\n", " for char in string:\n",
" if char == last_char:\n", " if char == prev_char:\n",
" count += 1\n", " count += 1\n",
" else:\n", " else:\n",
" compressed_string.append(last_char)\n", " result += prev_char + (str(count) if count > 1 else '')\n",
" compressed_string.append(str(count))\n", " prev_char = char\n",
" count = 1\n", " count = 1\n",
" last_char = char\n", " result += prev_char + str(count)\n",
" compressed_string.append(last_char)\n", " return result if len(result) < len(string) else string"
" compressed_string.append(str(count))\n",
"\n",
" # Convert the characters in the list to a string\n",
" return \"\".join(compressed_string)"
] ]
}, },
{ {
@ -167,7 +145,7 @@
" assert_equal(func(None), None)\n", " assert_equal(func(None), None)\n",
" assert_equal(func(''), '')\n", " assert_equal(func(''), '')\n",
" assert_equal(func('AABBCC'), 'AABBCC')\n", " assert_equal(func('AABBCC'), 'AABBCC')\n",
" assert_equal(func('AAABCCDDDD'), 'A3B1C2D4')\n", " assert_equal(func('AAABCCDDDD'), 'A3BC2D4')\n",
" print('Success: test_compress')\n", " print('Success: test_compress')\n",
"\n", "\n",
"\n", "\n",
@ -202,21 +180,21 @@
], ],
"metadata": { "metadata": {
"kernelspec": { "kernelspec": {
"display_name": "Python 2", "display_name": "Python 3",
"language": "python", "language": "python",
"name": "python2" "name": "python3"
}, },
"language_info": { "language_info": {
"codemirror_mode": { "codemirror_mode": {
"name": "ipython", "name": "ipython",
"version": 2 "version": 3
}, },
"file_extension": ".py", "file_extension": ".py",
"mimetype": "text/x-python", "mimetype": "text/x-python",
"name": "python", "name": "python",
"nbconvert_exporter": "python", "nbconvert_exporter": "python",
"pygments_lexer": "ipython2", "pygments_lexer": "ipython3",
"version": "2.7.10" "version": "3.4.3"
} }
}, },
"nbformat": 4, "nbformat": 4,

2
arrays_strings/compress/test_compress.py
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@ -7,7 +7,7 @@ class TestCompress(object):
assert_equal(func(None), None) assert_equal(func(None), None)
assert_equal(func(''), '') assert_equal(func(''), '')
assert_equal(func('AABBCC'), 'AABBCC') assert_equal(func('AABBCC'), 'AABBCC')
assert_equal(func('AAABCCDDDD'), 'A3B1C2D4') assert_equal(func('AAABCCDDDD'), 'A3BC2D4')
print('Success: test_compress') print('Success: test_compress')