Revert "Remove solution; revert later after going over the topic"

This reverts commit 8d50900eaa.

notes-and-examples/2026.08.27/topological_sort_solution.py

@@ -0,0 +1,47 @@
+from collections import deque
+
+# This is a solution which uses DFS to trace the paths; there
+# are other solutions as well
+
+classes = {
+    'CPSC 230': {'CPSC 231'},
+    'CPSC 231': {'CPSC 350', 'CPSC 330'},
+    'CPSC 350': {'CPSC 380', 'CPSC 408', 'CPSC 406'},
+    'CPSC 330': {'CPSC 351'},
+    'CPSC 351': set(),
+    'ENGR 101': set(),
+    'CPSC 380': set(),
+    'CPSC 406': set(),
+    'CPSC 408': set()
+}
+
+
+def topological_sort(adjacency_list: dict[str, set[str]]):
+    result = deque()
+    visited = set()
+
+    def dfs(current: str, visited_in_traversal=None):
+        if visited_in_traversal is None:
+            visited_in_traversal = set()
+        if current in visited:
+            return
+        if current in visited_in_traversal:
+            raise ValueError("Graph has a cycle")
+
+        visited_in_traversal.add(current)
+
+        for neighbor in adjacency_list[current]:
+            dfs(neighbor, visited_in_traversal)
+
+        # to trace the reverse path, use .append
+        result.appendleft(current)
+        visited.add(current)
+
+    for node in adjacency_list.keys():
+        if node not in visited:
+            dfs(node)
+
+    return result
+
+
+print(topological_sort(classes))