Update shallowest_decomp_tree.hpp

Author: lrvideckis

library/trees/shallowest_decomp_tree.hpp

@@ -23,7 +23,7 @@ void shallowest(auto& adj, auto f) {
   dfs(dfs, 0, 0);
   auto cpy(adj);
   adj.assign(sz(adj), {});
-  for (vi vec : order)
+  for (const vi& vec : order)
     for (int v : vec) {
       f(v);
       for (int u : cpy[v]) adj[u].push_back(v);