assert(res->pre_num == res->post_num);
res->pre_order = XMALLOCN(dfs_node_t*, res->pre_num);
res->post_order = XMALLOCN(dfs_node_t*, res->post_num);
assert(res->pre_num == res->post_num);
res->pre_order = XMALLOCN(dfs_node_t*, res->pre_num);
res->post_order = XMALLOCN(dfs_node_t*, res->post_num);