以 app 开头的有几个

词库是 apple、app、apply、banana。运行下面这段程序：

def build_trie(words):
    t = {}
    for w in words:
        node = t
        for ch in w:
            if ch not in node:
                node[ch] = {}
            node = node[ch]
        node["#"] = True
    return t

def walk(t, pre):
    node = t
    for ch in pre:
        if ch not in node:
            return None
        node = node[ch]
    return node

def collect(node, cur, out):
    if "#" in node:
        out.append(cur)
    for ch in node:
        if ch != "#":
            collect(node[ch], cur + ch, out)

t = build_trie(["apple", "app", "apply", "banana"])
out = []
collect(walk(t, "app"), "app", out)
print(len(out))