根据后序序列构建二叉树并输出其前序序列

#include<bits/stdc++.h>
using namespace std;
typedef struct TreeNode *Tree; //定义了一个叫做TreeNode的结构体指针 *Tree 
typedef char ElementType;

struct TreeNode{
    char val;
    Tree left;
    Tree right;
};
stack<char>sta;
Tree Creat()
{
    char c = sta.top();
    sta.pop();
    Tree T = (Tree)malloc(sizeof(struct TreeNode)); //创建新结点T 
    if(c=='.')T = NULL;
    else
    {   //右左根？ 
        T->right = Creat(); 
        T->left = Creat();
        T->val = c;
    }
    return T; //返回头节点T
}
void xx(Tree &t)
{
    cout<<t->val;
    if(t->left)xx(t->left);
    if(t->right)xx(t->right);
}
int main()
{
    string s;
    cin>>s;
    for(int i=0;i<s.length();i++)
        sta.push(s[i]); //用栈存储二叉树结点，可以实现从后往前遍历 
    Tree T = Creat(); //后序构造二叉树 
    xx(T); //先序遍历 
     return 0;
}