Added some combinatorics.

Author: calebmadrigal

combinatorics.py

@@ -0,0 +1,46 @@
+""" Various combinatorics functions. """
+
+__author__ = "Caleb Madriagl"
+__date__ = "2015-02-20"
+
+
+def factorial(n):
+    if n == 0:
+        return 1
+    else:
+        return n * factorial(n-1)
+
+
+def permutations(lst):
+    result = []
+
+    def permute(current, rest):
+        if rest == []:
+            result.append(current)
+            return
+        for r in rest:
+            permute(current + (r,), [i for i in rest if i != r])
+
+    permute((), lst)
+    return result
+
+
+def subsets(lst):
+    result = []
+
+    def _subsets(current, rest):
+        if rest == []:
+            result.append(current)
+            return
+
+        (first, *rest) = rest
+        _subsets(current + (first,), rest)
+        _subsets(current, rest)
+
+    _subsets((), lst)
+    return result
+
+if __name__ == "__main__":
+    print("Permutations of ['a','b','c']:", permutations(['a','b','c']))
+    print("Subsets of ['a','b','c']:", subsets(['a','b','c']))
+