检测链表中的环 · 实现

def has_cycle(____):
    def step(____):
        if i < ____ or i >= len(____):
            return -____
        return nxt[i]

    slow = ____
    fast = ____
    while True:
        slow = step(____)
        fast = step(____)
        fast = step(____)
        if slow < ____ or fast < ____:
            return ____
        if slow == fast:
            return ____

def solve():
    return has_cycle(____)