Solve problem 191.

python/e191.py

@@ -0,0 +1,38 @@
+from functools import lru_cache
+
+
+@lru_cache
+def count(days_left, absent_count):
+    if absent_count > 1:
+        return 0
+    if days_left == 0:
+        return 1
+    if days_left < 0:
+        return 0
+    c = 0
+    c += count(days_left - 1, absent_count + 1) # "a"
+    c += count(days_left - 2, absent_count + 1) # "la"
+    c += count(days_left - 3, absent_count + 1) # "lla"
+
+    c += count(days_left - 1, absent_count)     # "o"
+    c += count(days_left - 2, absent_count)     # "lo"
+    c += count(days_left - 3, absent_count)     # "llo"
+
+    if days_left == 2:
+        c += 1 # "ll"
+
+    if days_left == 1:
+        c += 1 # "l"
+
+    return c
+
+
+def euler_191():
+    return count(30, 0)
+
+
+if __name__ == "__main__":
+    solution = euler_191()
+    print("e191.py: " + str(solution))
+    assert(solution == 1918080160)
+