Added dijkstra.py

Author: wahreus

README.md

@@ -12,6 +12,7 @@ A repo for storing Python code and tracking learning progress. All programs are
 │   ├── binary_search.py
 │   ├── bubble_sort.py
 │   ├── dfs.py
+│   ├── dijkstra.py
 │   ├── heap_sort.py
 │   ├── merge_sort.py
 │   ├── quicksort.py

algorithms/dijkstra.py

@@ -0,0 +1,41 @@
+import heapq
+
+
+def dijkstra(graph: dict[str, list[tuple[str, int]]], start: str) -> dict[str, int]:
+    distances = {start: 0}
+    queue = [(0, start)]
+    while queue:
+        current_distance, current_node = heapq.heappop(queue)
+        if current_distance > distances[current_node]:
+            continue
+        for child, weight in graph[current_node]:
+            if weight < 0:
+                raise ValueError("Dijkstra's algorithm does not support negative weights.")
+            new_distance = current_distance + weight
+            if child not in distances or new_distance < distances[child]:
+                distances[child] = new_distance
+                heapq.heappush(queue, (new_distance, child))
+    return distances
+
+
+def main() -> None:
+    graph = {
+            "(A)": [("(B)", 4), ("(C)", 2)],
+            "(B)": [("(C)", 5), ("(D)", 10)],
+            "(C)": [("(E)", 3)],
+            "(D)": [],
+            "(E)": [("(D)", 4)],
+            }
+    start = "(A)"
+
+    distances = dijkstra(graph, start)
+    print(f"Shortest distances from {start}:")
+    for node in graph:
+        if node in distances:
+            print(f"- {node}: {distances[node]}")
+        else:
+            print(f"- {node}: unreachable")
+
+
+if __name__ == "__main__":
+    main()