n： 1, 2, 3, 4, 5, 6, 7
F(n): 1, 2, 2, 3, 3, 4, 4, 4, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7
Given n, return the nth number in F(n)

# my naive solution
def repeat(n):
    if n == 1:
        return [1]
    if n == 2:
        return [1, 2, 2]

    F = [1, 2, 2]

    for i in range(3, n+1):
        F += [i] * F[i-1]

    print(F, len(F))
    return F[n]

Follow up: If repeat() is called very frequently, how to optimize the performance? if n is very large, how to reduce memory usage?