TTree mytree;
mytree.MakeClass("mycode");

TList* glist = new TList();
glist->Add(g1);
glist->Add(g2);
file_out->Add(glist);

   TList *list = f1->GetListOfKeys();
   for (Int_t i=0;i<list->GetEntries();++i){
     TKey *key = (TKey*)list->At(i);
     TString name = key->GetName();
     TObject *o1 = f1->Get(name.Data());
     TString cname(o1->ClassName());
     if (!cname.Contains("TH1")) continue;
   }

TBox * b = new TBox(-25,-20,25,20);
//b->SetFillStyle(3003);
b->SetFillColorAlpha(10,0.0);
b->SetLineStyle(1);
b->SetLineColor(2);
b->SetLineWidth(10);
b->Draw();

.!ls
.! mv abc.txt def.txt
gSystem->Exec(“ls”);
gROOT->ProcessLine(Form(".!mkdir %s",filename.Data()));
gROOT->Macro("rootlogon.C");

std::vector<Double_t>* DiffusionVertexX;
tree->Branch("DiffusionVertexX", "vector<Double_t>", &DiffusionVertexX);

TChain should be created in advance of files to be added.

TChain * tree = new TChain("tree","tree");
tree->Add("/home/had/zhangce/g-2/MuYield/Root/MuYield_statistic_compain_1.root");
//...

Tree* t1  = TxtToTree(...);
Tree* t2  = TxtToTree(...);

TList *list = new TList;

	list->Add(t1);
	list->Add(t2);
	list->Add(t3);
        //...
        
TTree *newtree = TTree::MergeTrees(list);
newtree->SetName("newtree");

build a circle cut:

  const Int_t n = 30;
  Double_t x[n+1],y[n+1];

  Double_t rcut = 21;
  Double_t dphi = TMath::TwoPi()/n;

  for (Int_t i=0;i<n;i++) {
    x[i] = rcut*TMath::Cos(i*dphi);
    y[i] = rcut*TMath::Sin(i*dphi);
  }
  
  x[n] = x[0]; y[n] = y[0];
  TCutG *mcpcut = new TCutG("mcpcut",n+1,x,y);