无根树转有根树

std::vector<int> tree[maxn];
int fa[maxn];

void dfs(int u,int father) {
    int len = tree[u].size();
    for (int i = 0;i < len;i++) {
        int v = tree[u][i];
        if (v != father)
            dfs(v,fa[v] = u);
    }
}