Added an O(1) solution to problem 002 (TheAlgorithms#776)

* Added an O(1) solution to problem 002 * Removed comments from sol3.py that were accidentally added to sol4.py

Author: melonbreadjin

project_euler/problem_02/sol4.py

@@ -0,0 +1,13 @@
+importmath
+fromdecimalimport*
+
+getcontext().prec=100
+phi= (Decimal(5) **Decimal(0.5) +1) /Decimal(2)
+
+n=Decimal(int(input()) -1)
+
+index= (math.floor(math.log(n* (phi+2), phi) -1) //3) *3+2
+num=round(phi**Decimal(index+1)) / (phi+2)
+sum=num//2
+
+print(int(sum))