Two lists

You are given two lists $A$ and $B$ both containing the numbers $1 \dots n$ in some order. Your task is to count how many of the numbers $1 \dots n$ occur earlier on the list $A$ than on the list $B$.

In this task, $n$ can be large and an efficient algorithm is required. The time complexity should be $O(n)$.

In a file twolists.py, implement a function count that returns the desired count.

def count(a, b):
    # TODO

if __name__ == "__main__":
    print(count([2,3,4,1], [1,2,3,4])) # 3
    print(count([1,2,3,4], [1,2,3,4])) # 0
    print(count([4,7,3,1,6,2,5], [5,6,1,2,4,3,7])) # 3
    print(count([5,4,9,1,8,3,2,6,7], [6,2,8,4,9,1,5,7,3])) # 5

Explanation: In the first test, the numbers 2, 3 and 4 occur earlier on the list $A$ than on the list $B$.