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;
}