Implemented nth permutation function.

python/e024.py

@@ -14,13 +14,31 @@ def nth(generator, n):
 
 def nth_permutation(iterable, n):
     """ Returns the nth permutation of the iterable. """
-    #  XXX: Implement this!
-    return 0
+
+    def iterate_permutations(accumulator, iterable, count=[0]):
+        if not iterable:
+            if count[0] == n:
+                return accumulator
+            count[0] += 1
+            return False
+        for i in range(len(iterable)):
+            elem = iterable[i:i + 1]
+            rest = iterable[:i] + iterable[i + 1:]
+            p = iterate_permutations(accumulator + elem, rest)
+            if p:
+                return p
+        return False
+
+    return iterate_permutations([], iterable)
 
 
 def euler_024():
-    g = permutations("0123456789")
-    return int(nth(g, 1000000 - 1))
+    """ My first solution used a generator to get all permutations till
+    the desired one. My improved solution uses a function that returns only
+    the nth iteration which is the same algorithm, but does not required to
+    return all the permutations that are discarded anyway. """
+    return int("".join(nth_permutation(list("0123456789"), 1000000 - 1)))
+    #  return int(nth(permutations("0123456789"), 1000000 - 1))
 
 
 if __name__ == "__main__":

python/e036.py

@@ -0,0 +1,7 @@
+def euler_036():
+    return 0
+
+
+if __name__ == "__main__":
+    print("e036.py: {}".format(euler_036()))
+    assert(euler_036() == 55)