Solution to problem 12 part 1 in Python

src/Year_2021/P12.py

@@ -0,0 +1,160 @@
+# --- Day 12: Passage Pathing ---
+
+# With your submarine's subterranean subsystems subsisting suboptimally, the
+# only way you're getting out of this cave anytime soon is by finding a path
+# yourself. Not just a path - the only way to know if you've found the best
+# path is to find all of them.
+
+# Fortunately, the sensors are still mostly working, and so you build a rough
+# map of the remaining caves (your puzzle input). For example:
+
+# start-A
+# start-b
+# A-c
+# A-b
+# b-d
+# A-end
+# b-end
+
+# This is a list of how all of the caves are connected. You start in the cave
+# named start, and your destination is the cave named end. An entry like b-d
+# means that cave b is connected to cave d - that is, you can move between
+# them.
+
+# So, the above cave system looks roughly like this:
+
+#     start
+#     /   \
+# c--A-----b--d
+#     \   /
+#      end
+
+# Your goal is to find the number of distinct paths that start at start, end at
+# end, and don't visit small caves more than once. There are two types of
+# caves: big caves (written in uppercase, like A) and small caves (written in
+# lowercase, like b). It would be a waste of time to visit any small cave more
+# than once, but big caves are large enough that it might be worth visiting
+# them multiple times. So, all paths you find should visit small caves at most
+# once, and can visit big caves any number of times.
+
+# Given these rules, there are 10 paths through this example cave system:
+
+# start,A,b,A,c,A,end
+# start,A,b,A,end
+# start,A,b,end
+# start,A,c,A,b,A,end
+# start,A,c,A,b,end
+# start,A,c,A,end
+# start,A,end
+# start,b,A,c,A,end
+# start,b,A,end
+# start,b,end
+
+# (Each line in the above list corresponds to a single path; the caves visited
+# by that path are listed in the order they are visited and separated by
+# commas.)
+
+# Note that in this cave system, cave d is never visited by any path: to do so,
+# cave b would need to be visited twice (once on the way to cave d and a second
+# time when returning from cave d), and since cave b is small, this is not
+# allowed.
+
+# Here is a slightly larger example:
+
+# dc-end
+# HN-start
+# start-kj
+# dc-start
+# dc-HN
+# LN-dc
+# HN-end
+# kj-sa
+# kj-HN
+# kj-dc
+
+# The 19 paths through it are as follows:
+
+# start,HN,dc,HN,end
+# start,HN,dc,HN,kj,HN,end
+# start,HN,dc,end
+# start,HN,dc,kj,HN,end
+# start,HN,end
+# start,HN,kj,HN,dc,HN,end
+# start,HN,kj,HN,dc,end
+# start,HN,kj,HN,end
+# start,HN,kj,dc,HN,end
+# start,HN,kj,dc,end
+# start,dc,HN,end
+# start,dc,HN,kj,HN,end
+# start,dc,end
+# start,dc,kj,HN,end
+# start,kj,HN,dc,HN,end
+# start,kj,HN,dc,end
+# start,kj,HN,end
+# start,kj,dc,HN,end
+# start,kj,dc,end
+
+# Finally, this even larger example has 226 paths through it:
+
+# fs-end
+# he-DX
+# fs-he
+# start-DX
+# pj-DX
+# end-zg
+# zg-sl
+# zg-pj
+# pj-he
+# RW-he
+# fs-DX
+# pj-RW
+# zg-RW
+# start-pj
+# he-WI
+# zg-he
+# pj-fs
+# start-RW
+
+# How many paths through this cave system are there that visit small caves at
+# most once?
+
+from collections import defaultdict
+from typing import DefaultDict
+
+caves_map = defaultdict(list)
+with open("files/P12.txt") as f:
+    for row in f:
+        start, end = row.strip().split("-")
+        caves_map[start].append(end)
+        caves_map[end].append(start)
+
+
+def dfs(
+    node: str,
+    graph: DefaultDict[str, list[str]],
+    visited: set[str],
+    already_visited: bool,
+    counter: int = 0,
+):
+    if node == "end":
+        return 1
+    for neighbour in graph[node]:
+        if neighbour.isupper():
+            counter += dfs(neighbour, graph, visited, already_visited)
+        else:
+            if neighbour not in visited:
+                counter += dfs(
+                    neighbour, graph, visited | {neighbour}, already_visited
+                )
+            elif already_visited and neighbour not in {"start", "end"}:
+                counter += dfs(neighbour, graph, visited, False)
+    return counter
+
+
+def part_1() -> None:
+    ans = dfs("start", caves_map, {"start"}, already_visited=False)
+    print(f"There are {ans} paths visiting small caves")
+
+
+if __name__ == "__main__":
+    part_1()