Fix RuntimeError in bipartite-check DFS/BFS and clean up doctests (#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>

Author: 3 people

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()