采用先序法建立一棵二叉树，设计按先序输出二叉树的叶子，二叉树的数据域类型为字符型，扩展二叉树的叶子结点用‘#’表示，要求可以输出多棵二叉树的叶子结点，当二叉树为空时程序结束。

输入描述:

循环输入多棵扩展二叉树的先序遍历序列，每棵树占一行，以回车结束，每棵二叉树中结点之间以空格隔开

输出描述:

输出各二叉树中的叶子结点，每次输出后面都换行,当二叉树为空时，输出“NULL”，程序结束

输入样例:

A B # # C D # E # F # # G H # I K # # # #
A B D H # # I # # E J # # K # # C F L # # M # # G N # # O # #
#

输出样例:

B F K
H I J K L M N O
NULL

#include <iostream>
using namespace std;
struct BiNode
{
	char data;
	BiNode *lchild, *rchild;
};
class BiTree
{
public:
	BiTree() { root = creat(root); }
	void PreOrder() { PreOrder(root); }
	void PostOrder() { PostOrder(root); }
	void InOrder() { InOrder(root); }
	void LeverOrder() { LeverOrder(root); }
	int null()
	{
		int i = null(root);
		return i;
	}
private:
	BiNode *root;
	BiNode *creat(BiNode *bt);
	
	void PreOrder(BiNode *bt);
	void InOrder(BiNode *bt);
	void PostOrder(BiNode *bt);
	void LeverOrder(BiNode *bt);
	int null(BiNode *bt);

};
BiNode *BiTree::creat(BiNode *bt)
{
	char ch;
	cin >> ch;
	if(ch == '#')bt = NULL;
	else
	{
		bt = new BiNode;
		bt->data = ch;
		bt->lchild = creat(bt->lchild);
		bt->rchild = creat(bt->rchild);
	}
	return bt;
}

int BiTree::null(BiNode *bt)
{
	if(bt == NULL)
	{
		return 1;
	}
	else
	{
		return 0;
	}
}

void BiTree::PreOrder(BiNode *bt)
{
	if(bt == NULL)
		return;
	else
	{
		if(bt->lchild == NULL&&bt->rchild == NULL)
		{
			cout << bt->data << " ";
		}
		PreOrder(bt->lchild);
		PreOrder(bt->rchild);
	}
}

void BiTree::InOrder(BiNode *bt)
{
	if(bt == NULL)
		return;
	else
	{
		InOrder(bt->lchild);
		cout <<bt->data << " ";
		InOrder(bt->rchild);
	}
}

void BiTree::PostOrder(BiNode *bt)
{
	if(bt == NULL)
		return;
	else
	{
		PostOrder(bt->lchild);
		PostOrder(bt->rchild);
		cout <<bt->data << " ";
	}
}


void BiTree::LeverOrder(BiNode *q)
{
	int front,rear;
	BiNode *Q[20];
	front = rear = -1;
	if(root == NULL)return;
	Q[++rear] = root;
	while(front!=rear)
	{
		q = Q[++front];
		cout << q->data << " ";
		if(q->lchild!=NULL) Q[++rear] = q->lchild;
		if(q->rchild!=NULL) Q[++rear] = q->rchild;
	}
}

int main()
{
	while(1)
	{
		BiTree T;
		int i = T.null();
		if(i == 1)
		{
			cout << "NULL" << endl ;
			break;
		}
		else
		{
			T.PreOrder();
		}
		cout << endl;
	}
	return 0;
}