新手问一个关于根据先序,中序构建二叉树问题!!

public class Binarytree {
    BTNode root;
    int i;
    String s;
    public Binarytree(){
        String[] Pre="A B C I D E H F J G".split(" ");
        String[] Mid="B I C A H E J F G D".split(" ");
        root=BuildFromPreMid(Pre,Mid,0,Pre.length-1,0,Mid.length-1);
        lastorder(root);
    }

    public void preorder(BTNode tn){
        if(tn!=null){
            System.out.print(tn.S+" ");
            preorder(tn.left);
            preorder(tn.right);
        }
    }
    public void midorder(BTNode tn){
        if(tn!=null){
            midorder(tn.left);
            System.out.print(tn.S+" ");
            midorder(tn.right);
        }
    }
    public void lastorder(BTNode tn){
        if(tn!=null){
            lastorder(tn.left);
            lastorder(tn.right);
            System.out.print(tn.S+" ");
        }
    }
   
    BTNode BuildFromPreMid(String[] Pre,String[] Mid,int PS,int PE,int MS,int ME){
        BTNode node=new BTNode();
        node.S=Pre[PS];
        int i=0;
        while(!Mid[MS+i].equals(node.S)){
            i++;
        }
        if(i>0){
            node.left=BuildFromPreMid(Pre,Mid,PS+1,PS+i,MS,MS+i-1);
        }
        if(MS+i<ME){
            node.right=BuildFromPreMid(Pre,Mid,PS+i+1,PE,MS+i+1,ME);
        }
        return node;
    }

    public static void main(String[] args) {
        Binarytree b=new Binarytree();

    }
    class BTNode{
        String S;
        BTNode right,left,parent;
    }

}