前序、中序遍历转后序遍历

#include 
using namespace std;
struct TreeNode{
    char data;
    TreeNode*leftChild;
    TreeNode*rightChild;
    TreeNode(char c):data(c),leftChild(nullptr),rightChild(nullptr){};  //构造函数
};
TreeNode*Build(string preOrder,string inOrder){   //根据先序和中序序列建树
    if(preOrder.size()==0) return nullptr;          //递归终止
    char c=preOrder[0];
    TreeNode*root=new TreeNode(c);
    int position=inOrder.find(c);
    root->leftChild=Build(preOrder.substr(1,position),inOrder.substr(0,position));   //从1开始，长度是position
    root->rightChild=Build(preOrder.substr(position+1),inOrder.substr(position+1));  //从position+1开始一直到结束
    return root;
}
void PostOrder(TreeNode *root){  //后序遍历
    if(root==nullptr) return;
    PostOrder(root->leftChild);
    PostOrder(root->rightChild);
    printf("%c",root->data);
}
int main() {
    string preOrder,inOrder;
    while(cin>>preOrder>>inOrder){
        TreeNode *root=Build(preOrder,inOrder);
        PostOrder(root);
        cout<