Efficient algorithm for computing nth fibbonacci number

fibbonacci.py

```python3 
#code contributed by kirti purohit.
#time complexity O(n)
#which is much more efficient than the naive algorithm having a time complexity of O(2**n)
def fib(n,memo):
    result=0
    if memo[n] is not None:
        return memo[n]
    if n==1 or n==2:
        result=1
    else:
        result=fib(n-1,memo)+fib(n-2,memo)
    memo[n]=result
    return result


def fib_memo(n):
    memo = [None] * (n + 1)
    return fib(n, memo)

if __name__ == '__main__':
    n=int(input())
    print(fib_memo(n))
    
    ```