[Feature] Added permutations and combinations of strings.

Author: exterkamp

README.md

@@ -58,4 +58,6 @@ Different algorithmic programs.  Grouped by general topic.
 * [Factorial](algorithms/math/factorial.py)
 
 #### String Based Algorithms
-* [Fizz Buzz](algorithms/string/fizz_buzz.py)
+* [Fizz Buzz](algorithms/string/fizz_buzz.py)
+* [Permutations](algorithms/string/permutations.py)
+* [Combinations](algorithms/string/combinations.py)

algorithms/string/combinations.py

@@ -0,0 +1,12 @@
+def combinations_of_word(string: str):
+    """
+    Find all combinations of letters in a word.
+
+    """
+    output = []
+    for i in range(0,len(string)):
+        output.append(string[i])
+        for j in range(i+1, len(string)):
+            output.append(string[i:j+1])
+
+    return output

algorithms/string/permutations.py

@@ -0,0 +1,16 @@
+def permutations_of_word(string: str):
+    """
+    Find all permutations of letters in a word.
+
+    """
+    def permute(current, remaining):
+        if not remaining:
+            permute.list.append(current)
+            return
+        for i in range(0,len(remaining)):
+            remaining_minus = remaining[0:i] + remaining[i+1:]
+            permute(current + remaining[i], remaining_minus)
+
+    permute.list = []
+    permute('', list(string))
+    return permute.list

tests/test_combinations_word.py

@@ -0,0 +1,8 @@
+import unittest
+from algorithms.string.combinations import combinations_of_word
+
+class TestCombinationsWord(unittest.TestCase):
+
+    def test_combinations(self):
+        self.assertEqual(['t', 'tw', 'two', 'w', 'wo', 'o'], combinations_of_word('two'))
+        self.assertEqual(['1', '12', '123', '1234', '2', '23', '234', '3', '34', '4'], combinations_of_word('1234'))

tests/test_permutations_word.py

@@ -0,0 +1,8 @@
+import unittest
+from algorithms.string.permutations import permutations_of_word
+
+class TestPermuteWord(unittest.TestCase):
+
+    def test_permutation_word(self):
+        self.assertEqual(['two', 'tow', 'wto', 'wot', 'otw', 'owt'], permutations_of_word('two'))
+        self.assertEqual(['123', '132', '213', '231', '312', '321'], permutations_of_word('123'))