top = min(len(a1) - 1, len(a2) - 1)
if a2[top] <= a1[top]:
    return -1
while top > bottom:
    mid = top + bottom >> 1
if a2[mid] > a1[mid]:
    top = mid
else:
    bottom = mid + 1
return top
f1 = [(x + 20) for x in range(80)]
f2 = [(2 * x) for x in range(100)]
print(crossover(f1, f2))
Finding index where one sequence is greater than another