二叉树的操作

在计蒜客上学了二叉树，感觉自己还学了点东西，就贴在这里吧

#include<iostream>
#include<string>
using namespace std;
class Node {
public:
    char data;
    Node *lchild, *rchild;
    Node(int _data) {
        data = _data;
        lchild = NULL;
        rchild = NULL;
    }
    ~Node() {
        if (lchild != NULL) {
            delete lchild;
        }
        if (rchild != NULL) {
            delete rchild;
        }
    }
    void preorder(){   //先序遍历
        cout <<data<<" ";
        if(lchild!=NULL){
           lchild-> preorder();
        }
        if(rchild!=NULL){
           rchild-> preorder();
        }
    }
    void inorder(){   //中序遍历
       if(lchild!=NULL){
          lchild->inorder();
       }
        cout <<data<<" ";
        if(rchild!=NULL){
          rchild->inorder();
       }
    }
    void postorder() {   //后序遍历
        if (lchild != NULL) {
            lchild->postorder();
        }
        if (rchild != NULL) {
            rchild->postorder();
        }
        cout << data;
    }
    //已知先序和中序，求后序
    Node* build(const string &pre_str,const string &in_str,int len){
        Node* p=new Node(pre_str[0]-'0');
        int pos=in_str.find(pre_str[0]);
        if(pos>0){
            p->lchild=build(pre_str.substr(1,pos),in_str.substr(0,pos),pos);
        }
        if(len-pos-1>0){
           p->rchild=build(pre_str.substr(pos+1),in_str.substr(pos+1),len-pos-1);
        }
        return p;
    }
};
class BinaryTree {
private:
    Node *root;
public:
    BinaryTree() {
        root = NULL;
    }
    ~BinaryTree() {
        if (root != NULL) {
            delete root;
        }
    }
    BinaryTree(const string &pre_str,const string &in_str,int len){
       root=root->build(pre_str,in_str,len);
    }
    void preorder(){
       root->preorder();
    }
    void inorder(){
      root->inorder();
    }
    void postorder() {
        root->postorder();
    }
};
int main() {
    string pre_str;
    string in_str;
    cin>>pre_str;
    cin>>in_str;
    BinaryTree binarytree(pre_str,in_str,in_str.length());
    binarytree.postorder();
    cout <<endl;
    return 0;
}