Add doctests for cross function. Contributes to #9943 (#12991)

* Add doctests for cross function. Contributes to #9943

* Update sudoku_solver.py

---------

Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>

data_structures/arrays/sudoku_solver.py

@@ -11,6 +11,19 @@ import time
 def cross(items_a, items_b):
     """
     Cross product of elements in A and elements in B.
+
+    >>> cross('AB', '12')
+    ['A1', 'A2', 'B1', 'B2']
+    >>> cross('ABC', '123')
+    ['A1', 'A2', 'A3', 'B1', 'B2', 'B3', 'C1', 'C2', 'C3']
+    >>> cross('ABC', '1234')
+    ['A1', 'A2', 'A3', 'A4', 'B1', 'B2', 'B3', 'B4', 'C1', 'C2', 'C3', 'C4']
+    >>> cross('', '12')
+    []
+    >>> cross('A', '')
+    []
+    >>> cross('', '')
+    []
     """
     return [a + b for a in items_a for b in items_b]