3 files changed, 37 insertions, 1 deletions

diff --git a/2025/11/a.py b/2025/11/a.py
index 5fdb11b..cf06b6a 100644
--- a/2025/11/a.py
+++ b/2025/11/a.py
@@ -1,6 +1,12 @@
 import fileinput
 
+a = {}
 with fileinput.input() as lines:
         for line in lines:
-                ...
+                v, l2 = line[:-1].split(': ')
+                a[v] = l2.split(' ')
 
+def np(v):
+        return 1 if v == 'out' else sum(np(w) for w in a[v])
+
+print(np('you'))
diff --git a/2025/11/b.py b/2025/11/b.py
new file mode 100644
index 0000000..8fdcc8b
--- /dev/null
+++ b/2025/11/b.py
@@ -0,0 +1,16 @@
+import fileinput
+from functools import cache
+
+a = {}
+with fileinput.input() as lines:
+        for line in lines:
+                v, l2 = line[:-1].split(': ')
+                a[v] = l2.split(' ')
+
+@cache
+def np(v, d, f):
+        if v == 'out':
+                return 1 if d and f else 0
+        return sum(np(w, d or v == 'dac', f or v == 'fft') for w in a[v])
+
+print(np('svr', False, False))
diff --git a/2025/README.md b/2025/README.md
index 09d97c1..951213c 100644
--- a/2025/README.md
+++ b/2025/README.md
@@ -11,6 +11,7 @@ Example
 
 ```
 Day   -Part 1-   -Part 2-
+ 11   00:14:22   00:21:19
  10   00:27:43   11:51:51
   9   00:05:05   02:11:41
   8   00:29:14   00:33:02
@@ -213,3 +214,16 @@ AoC problem (see `../2023/24/24b.c`).
 
 This time I left some comments in the code, so check out `10/b.py` if
 you want to know the details.
+
+### Day 11: Reactor
+
+This is very easy, at least if you have ever worked with graphs. For
+part 1I have implemented a recursive function `np(v)` that counts the
+number of paths from a node `v` to `out`: it returns 1 if `v == out`,
+or the sum of `np(w)` for all neighbors `w` of `v` otherwise.
+
+For part 2 the function takes 2 extra parameters that denote whether
+or not we have passed through the two required intermediate nodes.
+
+The paths in part 1 are small enough that memoization is not required,
+but in part 2 we need to cache the intermediate results.