Improve Project Euler problem 014 solution 2 (#5744)

* Improve solution

* Uncomment code that has been commented due to slow execution affecting Travis

* Fix

* scikit-fuzzy is causing broken builds

* fuzz = None

* Update fuzzy_operations.py

Co-authored-by: Christian Clauss <cclauss@me.com>

project_euler/problem_014/sol2.py

@@ -27,25 +27,27 @@ Which starting number, under one million, produces the longest chain?
 """
 from __future__ import annotations
 
+COLLATZ_SEQUENCE_LENGTHS = {1: 1}
+
 
 def collatz_sequence_length(n: int) -> int:
     """Returns the Collatz sequence length for n."""
-    sequence_length = 1
-    while n != 1:
-        if n % 2 == 0:
-            n //= 2
-        else:
-            n = 3 * n + 1
-        sequence_length += 1
+    if n in COLLATZ_SEQUENCE_LENGTHS:
+        return COLLATZ_SEQUENCE_LENGTHS[n]
+    if n % 2 == 0:
+        next_n = n // 2
+    else:
+        next_n = 3 * n + 1
+    sequence_length = collatz_sequence_length(next_n) + 1
+    COLLATZ_SEQUENCE_LENGTHS[n] = sequence_length
     return sequence_length
 
 
 def solution(n: int = 1000000) -> int:
     """Returns the number under n that generates the longest Collatz sequence.
 
-    # The code below has been commented due to slow execution affecting Travis.
-    # >>> solution(1000000)
-    # 837799
+    >>> solution(1000000)
+    837799
     >>> solution(200)
     171
     >>> solution(5000)