设计一个算法将二叉树的叶结点从左到右的顺序连成一个单链表，表头指针为head。二叉树按二叉链表方式存储，链接时用叶结点的右指针来存放单链表指针。

#include <iostream>
#include <stack>
#include <queue>
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');
    root->right->left= buytreenode('F');
    root->right->right= buytreenode('G');
    root->left->left->left= buytreenode('H');
    root->left->left->right= buytreenode('I');
    return root;
}

ptreenode build_tree2()
{
    ptreenode root= buytreenode('A');
    root->left= buytreenode('B');
    root->right= buytreenode('C');
    root->left->left= buytreenode('D');
    root->left->right= buytreenode('E');
    root->right->left= buytreenode('F');
    root->right->right= buytreenode('G');
    root->left->left->left= buytreenode('H');
    root->left->left->right= buytreenode('I');
    root->left->right->left= buytreenode('J');
    root->left->right->right= buytreenode('K');
    root->right->right->left= buytreenode('L');
    root->right->right->right= buytreenode('M');
    return root;
}

ptreenode build_tree3()
{
    ptreenode root= buytreenode('A');
    root->left= buytreenode('B');
    root->right= buytreenode('C');
    root->left->left= buytreenode('D');
    root->left->right= buytreenode('E');
    root->right->left= buytreenode('F');
    root->right->right= buytreenode('G');
    root->left->left->left= buytreenode('H');
    root->left->left->right= buytreenode('I');
    root->left->right->left= buytreenode('J');
    root->left->right->right= buytreenode('K');
    root->right->left->left= buytreenode('L');
    root->right->left->right= buytreenode('M');
    root->right->right->left= buytreenode('N');
    root->right->right->right= buytreenode('O');
    return root;
}

void print_tree(ptreenode root) {
    std::queue<ptreenode> tmp;
    tmp.push(root);
    int s = tmp.size();
    while (!tmp.empty()) {
        ptreenode t = tmp.front();
        tmp.pop();
        s--;
        printf("%3c", t->data);
        if (t->left) tmp.push(t->left);
        if (t->right) tmp.push(t->right);
        if (s == 0) puts(""), s = tmp.size();
    }
}

void _linkleaf(ptreenode root ,ptreenode &head,ptreenode &pre)
{
    if(root== nullptr) return;
    _linkleaf(root->left,head,pre);
    if(!root->left&&!root->right)//叶子结点
    {
        if(head== nullptr)head=root,pre=root;
        else pre->right=root,pre=pre->right;
    }
    _linkleaf(root->right,head,pre);
}
ptreenode linkleaf(ptreenode root)
{
    ptreenode head=(ptreenode) malloc(sizeof (ptreenode));
    ptreenode pre= (ptreenode) malloc(sizeof (ptreenode));
    pre= nullptr,head= nullptr;
    _linkleaf(root,head,pre);
    return head;
}
int main() {

    ptreenode root=build_tree();
    print_tree(root);
    ptreenode tmp= linkleaf(root);
    printf("the leaves' link is:");
    for(ptreenode i=tmp;i;i=i->right) printf("%3c",i->data);
    puts("");


    root=build_tree2();
    print_tree(root);
    tmp= linkleaf(root);
    printf("the leaves' link is:");
    for(ptreenode i=tmp;i;i=i->right) printf("%3c",i->data);
    puts("");

    root=build_tree3();
    print_tree(root);
    tmp= linkleaf(root);
    printf("the leaves' link is:");
    for(ptreenode i=tmp;i;i=i->right) printf("%3c",i->data);
    puts("");
    return 0;
}