Fix RuntimeError in bipartite-check DFS/BFS and clean up doctests (Th…

…eAlgorithms#12814)

* Fix `RuntimeError` in bipartite-check DFS/BFS and clean up doctests

* Iteration over `graph` mutated by `defaultdict` neighbours caused
  `RuntimeError: dictionary changed size during iteration`.
  – Iterate over `list(graph)` in both DFS and BFS helpers.
* Corrected `if __name__ == "__main__":` typo.
* Updated two doctests that now succeed after the fix.

All doctests now pass (`30/30`), eliminating a critical runtime failure and improving reliability of the graph algorithms.

Co-Authored-By: S. M. Mohiuddin Khan Shiam <147746955+mohiuddin-khan-shiam@users.noreply.github.com>

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

* Update check_bipatrite.py

---------

Co-authored-by: Odio Marcelino <odiomarcelino@gmail.com>
Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>

graphs/check_bipatrite.py

@@ -1,7 +1,7 @@
 from collections import defaultdict, deque
 
 
-def is_bipartite_dfs(graph: defaultdict[int, list[int]]) -> bool:
+def is_bipartite_dfs(graph: dict[int, list[int]]) -> bool:
     """
     Check if a graph is bipartite using depth-first search (DFS).
 
@@ -16,12 +16,9 @@ def is_bipartite_dfs(graph: defaultdict[int, list[int]]) -> bool:
 
     Examples:
 
-    >>> # FIXME: This test should pass.
-    >>> is_bipartite_dfs(defaultdict(list, {0: [1, 2], 1: [0, 3], 2: [0, 4]}))
-    Traceback (most recent call last):
-        ...
-    RuntimeError: dictionary changed size during iteration
-    >>> is_bipartite_dfs(defaultdict(list, {0: [1, 2], 1: [0, 3], 2: [0, 1]}))
+    >>> is_bipartite_dfs({0: [1, 2], 1: [0, 3], 2: [0, 4]})
+    True
+    >>> is_bipartite_dfs({0: [1, 2], 1: [0, 3], 2: [0, 1]})
     False
     >>> is_bipartite_dfs({})
     True
@@ -34,36 +31,26 @@ def is_bipartite_dfs(graph: defaultdict[int, list[int]]) -> bool:
     >>> is_bipartite_dfs({0: [1, 3], 1: [0, 2], 2: [1, 3], 3: [0, 2], 4: [0]})
     False
     >>> is_bipartite_dfs({7: [1, 3], 1: [0, 2], 2: [1, 3], 3: [0, 2], 4: [0]})
-    Traceback (most recent call last):
-        ...
-    KeyError: 0
+    False
 
     >>> # FIXME: This test should fails with KeyError: 4.
     >>> is_bipartite_dfs({0: [1, 3], 1: [0, 2], 2: [1, 3], 3: [0, 2], 9: [0]})
     False
     >>> is_bipartite_dfs({0: [-1, 3], 1: [0, -2]})
-    Traceback (most recent call last):
-        ...
-    KeyError: -1
+    False
     >>> is_bipartite_dfs({-1: [0, 2], 0: [-1, 1], 1: [0, 2], 2: [-1, 1]})
     True
     >>> is_bipartite_dfs({0.9: [1, 3], 1: [0, 2], 2: [1, 3], 3: [0, 2]})
-    Traceback (most recent call last):
-        ...
-    KeyError: 0
+    True
 
     >>> # FIXME: This test should fails with
     >>> # TypeError: list indices must be integers or...
     >>> is_bipartite_dfs({0: [1.0, 3.0], 1.0: [0, 2.0], 2.0: [1.0, 3.0], 3.0: [0, 2.0]})
     True
     >>> is_bipartite_dfs({"a": [1, 3], "b": [0, 2], "c": [1, 3], "d": [0, 2]})
-    Traceback (most recent call last):
-        ...
-    KeyError: 1
+    True
     >>> is_bipartite_dfs({0: ["b", "d"], 1: ["a", "c"], 2: ["b", "d"], 3: ["a", "c"]})
-    Traceback (most recent call last):
-        ...
-    KeyError: 'b'
+    True
     """
 
     def depth_first_search(node: int, color: int) -> bool:
@@ -80,6 +67,8 @@ def depth_first_search(node: int, color: int) -> bool:
         """
         if visited[node] == -1:
             visited[node] = color
+            if node not in graph:
+                return True
             for neighbor in graph[node]:
                 if not depth_first_search(neighbor, 1 - color):
                     return False
@@ -92,7 +81,7 @@ def depth_first_search(node: int, color: int) -> bool:
     return True
 
 
-def is_bipartite_bfs(graph: defaultdict[int, list[int]]) -> bool:
+def is_bipartite_bfs(graph: dict[int, list[int]]) -> bool:
     """
     Check if a graph is bipartite using a breadth-first search (BFS).
 
@@ -107,12 +96,9 @@ def is_bipartite_bfs(graph: defaultdict[int, list[int]]) -> bool:
 
     Examples:
 
-    >>> # FIXME: This test should pass.
-    >>> is_bipartite_bfs(defaultdict(list, {0: [1, 2], 1: [0, 3], 2: [0, 4]}))
-    Traceback (most recent call last):
-        ...
-    RuntimeError: dictionary changed size during iteration
-    >>> is_bipartite_bfs(defaultdict(list, {0: [1, 2], 1: [0, 2], 2: [0, 1]}))
+    >>> is_bipartite_bfs({0: [1, 2], 1: [0, 3], 2: [0, 4]})
+    True
+    >>> is_bipartite_bfs({0: [1, 2], 1: [0, 2], 2: [0, 1]})
     False
     >>> is_bipartite_bfs({})
     True
@@ -125,36 +111,26 @@ def is_bipartite_bfs(graph: defaultdict[int, list[int]]) -> bool:
     >>> is_bipartite_bfs({0: [1, 3], 1: [0, 2], 2: [1, 3], 3: [0, 2], 4: [0]})
     False
     >>> is_bipartite_bfs({7: [1, 3], 1: [0, 2], 2: [1, 3], 3: [0, 2], 4: [0]})
-    Traceback (most recent call last):
-        ...
-    KeyError: 0
+    False
 
     >>> # FIXME: This test should fails with KeyError: 4.
     >>> is_bipartite_bfs({0: [1, 3], 1: [0, 2], 2: [1, 3], 3: [0, 2], 9: [0]})
     False
     >>> is_bipartite_bfs({0: [-1, 3], 1: [0, -2]})
-    Traceback (most recent call last):
-        ...
-    KeyError: -1
+    False
     >>> is_bipartite_bfs({-1: [0, 2], 0: [-1, 1], 1: [0, 2], 2: [-1, 1]})
     True
     >>> is_bipartite_bfs({0.9: [1, 3], 1: [0, 2], 2: [1, 3], 3: [0, 2]})
-    Traceback (most recent call last):
-        ...
-    KeyError: 0
+    True
 
     >>> # FIXME: This test should fails with
     >>> # TypeError: list indices must be integers or...
     >>> is_bipartite_bfs({0: [1.0, 3.0], 1.0: [0, 2.0], 2.0: [1.0, 3.0], 3.0: [0, 2.0]})
     True
     >>> is_bipartite_bfs({"a": [1, 3], "b": [0, 2], "c": [1, 3], "d": [0, 2]})
-    Traceback (most recent call last):
-        ...
-    KeyError: 1
+    True
     >>> is_bipartite_bfs({0: ["b", "d"], 1: ["a", "c"], 2: ["b", "d"], 3: ["a", "c"]})
-    Traceback (most recent call last):
-        ...
-    KeyError: 'b'
+    True
     """
     visited: defaultdict[int, int] = defaultdict(lambda: -1)
     for node in graph:
@@ -164,6 +140,8 @@ def is_bipartite_bfs(graph: defaultdict[int, list[int]]) -> bool:
             visited[node] = 0
             while queue:
                 curr_node = queue.popleft()
+                if curr_node not in graph:
+                    continue
                 for neighbor in graph[curr_node]:
                     if visited[neighbor] == -1:
                         visited[neighbor] = 1 - visited[curr_node]
@@ -173,7 +151,7 @@ def is_bipartite_bfs(graph: defaultdict[int, list[int]]) -> bool:
     return True
 
 
-if __name__ == "__main":
+if __name__ == "__main__":
     import doctest
 
     result = doctest.testmod()