Optimization for shell sort (#4038)

* fixed shell sort

* udpate code style

* Update sorts/shell_sort.py

Co-authored-by: John Law <[email protected]>

Co-authored-by: John Law <[email protected]>

Author: realDuYuanChao

Diff for: sorts/shell_sort.py

@@ -1,13 +1,5 @@
 """
-This is a pure Python implementation of the shell sort algorithm
-
-For doctests run following command:
-python -m doctest -v shell_sort.py
-or
-python3 -m doctest -v shell_sort.py
-
-For manual testing run:
-python shell_sort.py
+https://en.wikipedia.org/wiki/Shellsort#Pseudocode
 """
 
 
@@ -19,26 +11,29 @@ def shell_sort(collection):
 
     >>> shell_sort([0, 5, 3, 2, 2])
     [0, 2, 2, 3, 5]
-
     >>> shell_sort([])
     []
-
     >>> shell_sort([-2, -5, -45])
     [-45, -5, -2]
     """
     # Marcin Ciura's gap sequence
-    gaps = [701, 301, 132, 57, 23, 10, 4, 1]
 
+    gaps = [701, 301, 132, 57, 23, 10, 4, 1]
     for gap in gaps:
         for i in range(gap, len(collection)):
+            insert_value = collection[i]
             j = i
-            while j >= gap and collection[j] < collection[j - gap]:
-                collection[j], collection[j - gap] = collection[j - gap], collection[j]
+            while j >= gap and collection[j - gap] > insert_value:
+                collection[j] = collection[j - gap]
                 j -= gap
+            collection[j] = insert_value
     return collection
 
 
 if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
     user_input = input("Enter numbers separated by a comma:\n").strip()
     unsorted = [int(item) for item in user_input.split(",")]
     print(shell_sort(unsorted))