Add solution to 2024-12-16

2024/day16/solutions.py

@@ -0,0 +1,43 @@
+import networkx as nx
+
+with open("input") as f:
+    ls = f.read().strip().split("\n")
+
+fourdir = (1, -1, 1j, -1j)
+
+G = nx.DiGraph()
+
+for i, l in enumerate(ls):
+    for j, x in enumerate(l):
+        if x == "#":
+            continue
+        z = i + 1j * j
+        if x == "S":
+            start = (z, 1j)
+        if x == "E":
+            end = z
+        for dz in fourdir:
+            G.add_node((z, dz))
+
+for z, dz in G.nodes:
+    if (z + dz, dz) in G.nodes:
+        G.add_edge((z, dz), (z + dz, dz), weight=1)
+    for rot in -1j, 1j:
+        G.add_edge((z, dz), (z, dz * rot), weight=1000)
+
+for dz in fourdir:
+    G.add_edge((end, dz), "end", weight=0)
+
+# Part 1
+print(nx.shortest_path_length(G, start, "end", weight="weight"))
+
+# Part2
+print(
+    len(
+        {
+            z
+            for path in nx.all_shortest_paths(G, start, "end", weight="weight")
+            for z, _ in path[:-1]
+        }
+    )
+)