假设二叉树采用二叉链存储结构存储，设计一个算法，求先序遍历序列中第k个结点的值

#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;
}

ptreenode printnode(ptreenode root,int &x)
{
    if(root== nullptr) return root;
    if (x--==1) return root;
    ptreenode ch=printnode(root->left,x);
    if(ch) return ch;
    ch=printnode(root->right,x);
    return ch;
}
void print_prior(ptreenode root)
{
    if(root== nullptr) return ;
    printf("%3c",root->data);
    print_prior(root->left);
    print_prior(root->right);
}
int main() {
    ptreenode root=build_tree();
    print_prior(root);
    puts("");
    int number=0;
    for(int i=1;i<=5;i++)
    {
        number=i;
        printf("the %2dth element is:%3c",i,printnode(root,number)->data);
        puts("");
    }
    return 0;
}