如果 main 一次都没动过呢

同一张图，但假设 main 还停在分叉点 7b4e2d0 上。运行下面这段程序：

def ancestors(g, c):
    seen = set()
    st = [c]
    while st:
        x = st.pop()
        if x in seen:
            continue
        seen.add(x)
        st.extend(g[x])
    return seen

def is_ff(g, cur, target):
    return cur in ancestors(g, target)

g = {'3f2a91c': [],
     '7b4e2d0': ['3f2a91c'],
     'a1c5f83': ['7b4e2d0'],
     'e90d417': ['7b4e2d0'],
     '5c8b206': ['e90d417']}
print(is_ff(g, '7b4e2d0', '5c8b206'))