为啥点分治的重心写成这样？

每次获取重心后，更新父亲结点的 size？

void getroot(int u, int fa) {
    int mxs = 0;
    sz[u] = 1;
    for (int i = head[u]; i; i = e[i].next) {
        int to = e[i].to;
        if (to == fa || del[to]) continue;
        getroot(to, u);
        if (root != -1) return;
        sz[u] += sz[to];
        mxs = max(mxs, sz[to]);
    }
    mxs = max(mxs, sum - sz[u]);
    if (mxs * 2 <= sum) {
        root = u;
        sz[fa] = sum - sz[u];
    }
}