编写后序遍历二叉树的非递归算法

#include <iostream>
#include <stack>
typedef struct treenode{
    char data;
    struct treenode *left;
    struct treenode *right;
}treenode,*ptreenode;

ptreenode buytreenode(char x)
{
    ptreenode n=(ptreenode) malloc(sizeof (treenode));
    n->data=x;
    n->left= nullptr,n->right= nullptr;
    return n;
}
ptreenode build_tree()
{
    ptreenode root= buytreenode('A');
    root->left= buytreenode('B');
    root->right= buytreenode('C');
    root->left->left= buytreenode('D');
    root->left->right= buytreenode('E');
    return root;
}

void postorder(ptreenode root)
{
    std::stack<ptreenode> record;
    ptreenode pointer=root,pre= nullptr;
    while(pointer || !record.empty())
    {
        if(pointer) record.push(pointer),pointer=pointer->left;
        else{
            pointer=record.top();
            if(pointer->right&&pointer->right!=pre) pointer=pointer->right;
            else{
                printf("%3c",record.top()->data);
                record.pop();
                pre=pointer;
                pointer= nullptr;
            }
        }
    }
}
int main() {
    ptreenode root=build_tree();
    postorder(root);
    return 0;
}