circular array

def cycle(a):
    n = len(a)
    idx = 0
    if a[idx] == 0:
        return len(a) == 1
    for i in range(n):
        idx = (idx + a[idx]) % n
    return idx == 0
    
assert cycle([0]) == True
assert cycle([0, 0]) == False
assert cycle([0, 1]) == False
assert cycle([1, 0]) == False
assert cycle([1, 1]) == True
assert cycle([1, -1]) == True
assert cycle([2, 2, 2]) == True
assert cycle([2, 2, 1]) == False
assert cycle([1, 2, 0]) == False
assert cycle([2, 2, 0]) == False
assert cycle([2, 2, -1]) == True