tasks a-s

Task/Permutations-by-swapping/Python/permutations-by-swapping-3.py

@@ -0,0 +1,17 @@
+def s_permutations(seq):
+    items = [[]]
+    for j in seq:
+        new_items = []
+        for i, item in enumerate(items):
+            if i % 2:
+                # step up
+                new_items += [item[:i] + [j] + item[i:]
+                              for i in range(len(item) + 1)]
+            else:
+                # step down
+                new_items += [item[:i] + [j] + item[i:]
+                              for i in range(len(item), -1, -1)]
+        items = new_items
+
+    return [(tuple(item), -1 if i % 2 else 1)
+            for i, item in enumerate(items)]