Added notebook solving the following: Check if a string is a permutation of another.

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Donne Martin 2015-05-02 17:06:21 -04:00
parent 3f23c6ee9e
commit 199157aeed
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@ -7,6 +7,7 @@ Data structures and algorithms practice problems.
* [Check if a string contains unique characters](http://nbviewer.ipython.org/github/donnemartin/practice/blob/master/arrays-strings/unique_chars.ipynb) * [Check if a string contains unique characters](http://nbviewer.ipython.org/github/donnemartin/practice/blob/master/arrays-strings/unique_chars.ipynb)
* [Reverse characters in a string](http://nbviewer.ipython.org/github/donnemartin/practice/blob/master/arrays-strings/reverse_string.ipynb) * [Reverse characters in a string](http://nbviewer.ipython.org/github/donnemartin/practice/blob/master/arrays-strings/reverse_string.ipynb)
* [Check if a string is a permutation of another](http://nbviewer.ipython.org/github/donnemartin/practice/blob/master/arrays-strings/permutation.ipynb)
## License ## License

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@ -0,0 +1,171 @@
{
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{
"cell_type": "markdown",
"metadata": {},
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"## Problem: Determine if a string is a permutation of another string\n",
"\n",
"* [Clarifying Questions](#Clarifying-Questions)\n",
"* [Test Cases](#Test-Cases)\n",
"* [Algorithm: Compare Sorted Strings](#Algorithm:-Compare-Sorted-Strings)\n",
"* [Code: Compare Sorted Strings](#Code:-Compare-Sorted-Strings)\n",
"* [Pythonic-Code](#Pythonic-Code)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Clarifying Questions\n",
"\n",
"* Is the string ASCII (extended)? Or Unicode?\n",
" * ASCII extended, which is 256 characters\n",
"* Is whitespace important?\n",
" * Yes\n",
"* Is this case sensitive? 'Nib', 'bin' is not a match?\n",
" * Yes"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Test Cases\n",
"\n",
"* One or more empty strings -> False\n",
"* 'Nib', 'bin' -> False\n",
"* 'act', 'cat' -> True\n",
"* 'a ct', 'ca t' -> True"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Algorithm: Compare Sorted Strings\n",
"\n",
"Anagrams contain the same strings but in different orders. This approach could be slow for large strings due to sorting.\n",
"\n",
"* Sort both strings\n",
"* If both sorted strings are equal\n",
" * return True\n",
"* Else\n",
" * return False\n",
"\n",
"Complexity:\n",
"* Time: O(n log n) from the sort, in general\n",
"* Space: Additional O(l + m) is created by the sorting algorithm, where l is the length of one string and m is the length of the other"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Code: Compare Sorted Strings"
]
},
{
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"execution_count": null,
"metadata": {
"collapsed": false
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"outputs": [],
"source": [
"def permutations(str1, str2):\n",
" return sorted(str1) == sorted(str2)\n",
"\n",
"print(permutations('', 'foo'))\n",
"print(permutations('Nib', 'bin'))\n",
"print(permutations('act', 'cat'))\n",
"print(permutations('a ct', 'ca t'))"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Algorithm: Hash Map Lookup\n",
"\n",
"We'll keep a hash map (dict) to keep track of characters we encounter. \n",
"\n",
"Steps:\n",
"* Scan each character\n",
"* For each character in each string:\n",
" * If the character does not exist in a hash map, add the character to a hash map\n",
" * Else, increment the character's count\n",
"* If the hash maps for each string are equal\n",
" * Return True\n",
"* Else\n",
" * Return False\n",
"\n",
"Notes:\n",
"* Since the characters are in ASCII, we could potentially use an array of size 128 (or 256 for extended ASCII)\n",
"* Instead of using two hash maps, you could use one hash map and increment character values based on the first string and decrement based on the second string\n",
"* You can short circuit if the lengths of each string are not equal, len() in Python is generally O(1)\n",
"\n",
"Complexity:\n",
"* Time: O(n)\n",
"* Space: Additional O(m), where m is the number of unique characters in the hash map"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Code"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {
"collapsed": false
},
"outputs": [],
"source": [
"from collections import defaultdict\n",
"\n",
"def unique_counts(string):\n",
" dict_chars = defaultdict(int)\n",
" for char in string:\n",
" dict_chars[char] += 1\n",
" return dict_chars\n",
"\n",
"def permutations(str1, str2):\n",
" if len(str1) != len(str2):\n",
" return False\n",
" unique_counts1 = unique_counts(str1)\n",
" unique_counts2 = unique_counts(str2)\n",
" return unique_counts1 == unique_counts2\n",
"\n",
"print(permutations('', 'foo'))\n",
"print(permutations('Nib', 'bin'))\n",
"print(permutations('act', 'cat'))\n",
"print(permutations('a ct', 'ca t'))"
]
}
],
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