tasks a-s

Task/Permutations-by-swapping/Python/permutations-by-swapping-2.py

@@ -0,0 +1,19 @@
+def s_permutations(seq):
+    def s_perm(seq):
+        if not seq:
+            return [[]]
+        else:
+            new_items = []
+            for i, item in enumerate(s_perm(seq[:-1])):
+                if i % 2:
+                    # step up
+                    new_items += [item[:i] + seq[-1:] + item[i:]
+                                  for i in range(len(item) + 1)]
+                else:
+                    # step down
+                    new_items += [item[:i] + seq[-1:] + item[i:]
+                                  for i in range(len(item), -1, -1)]
+            return new_items
+
+    return [(tuple(item), -1 if i % 2 else 1)
+            for i, item in enumerate(s_perm(seq))]