Fibonacci Number — Solution & Editorial

Back to the Problem

Iterate with two variables — O(n), no recursion needed (naive recursion is exponential and will TLE conceptually, though n ≤ 80 mainly punishes 32-bit overflow). Use 64-bit integers in C/C++/Java; Python is unbounded.

a, b = 0, 1
for _ in range(int(input())):
    a, b = b, a + b
print(a)