Add Extra edge cases (#12995)

* Add Extra edge cases

* Update coloring.py

---------

Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>
Co-authored-by: cclauss <3709715+cclauss@users.noreply.github.com>

Author: 3 people

backtracking/coloring.py

@@ -104,6 +104,14 @@ def color(graph: list[list[int]], max_colors: int) -> list[int]:
     >>> max_colors = 2
     >>> color(graph, max_colors)
     []
+    >>> color([], 2)  # empty graph
+    []
+    >>> color([[0]], 1)  # single node, 1 color
+    [0]
+    >>> color([[0, 1], [1, 0]], 1)  # 2 nodes, 1 color (impossible)
+    []
+    >>> color([[0, 1], [1, 0]], 2)  # 2 nodes, 2 colors (possible)
+    [0, 1]
     """
     colored_vertices = [-1] * len(graph)