问题：

如何实现二叉树的层次遍历？

解析：

我们可以使用队列来解决这个问题

<1>将根节点压入队列

<2>判断队列是否为空

<3>不为空则获取队列最前端的元素，打印出该元素

<4>将该元素移除队列

<5>如果该元素有左孩子，则将其左孩子进入队列

<6>如果该元素有右孩子，则将其右孩子进入队列

主要函数如下所示：

template<typename Type> void BSTrees<Type>::traverseLevel(Node* root)
{
	if( root == NULL)
	{
		cout<<"This is a empty tree"<<endl;
		return;
	}
	queue<Node*> que;
	que.push(root);
	while( !que.empty() )
	{
		Node* ptr = que.front();
		que.pop();
		cout<<ptr->e<<endl;
		if(ptr->leftChild != NULL)
			que.push(ptr->leftChild);
		if(ptr->rightChild != NULL)
			que.push(ptr->rightChild);
	}
}

//BSTrees.h
#ifndef DDXX_BSTREES_H
#define DDXX_BSTREES_H
#include <iostream>
#include <queue>
using namespace std;
template<typename Type>
class BSTrees
{
public:
	BSTrees();
	~BSTrees();
public:
	struct Node
	{
		Type	e;
		Node*	leftChild;
		Node*	rightChild;
		Node()
		{
		}
		Node(Type _e)
		{
			e			= _e;
			leftChild	= NULL;
			rightChild	= NULL;
		}
	};
public:
	bool	insert(Type e);
	bool	erase1(Type e);
	bool	erase2(Type e);
	void	clear();
	bool	isEmpty();
	int		getLength();
	Node*	find(Type e);
	Node*	getRoot();
	Node*	getParent(Node* p);
	void	traverse(Node* ptr);
	void	traverseLevel(Node* ptr);
private:
	void	clears(Node* &p);
private:
	Node*	m_root;
	int		m_Length;
};

template<typename Type> BSTrees<Type>::BSTrees()
{
	m_root = NULL;
	m_Length = 0;
}

template<typename Type> bool BSTrees<Type>::insert(Type e)
{
	Node* ptr = new Node(e);
	if ( ptr == NULL )
	{
		cout<<"Allocate memory for the element failed"<<endl;
		return false;
	}

	if( m_root == NULL)
	{
		m_root = ptr;
		m_Length++;
		return true;
	}

	Node* p			= m_root;
	Node* pParent	= m_root;
	while( p != NULL)
	{
		if( e == p->e)
		{
			cout<<"the element is already exist"<<endl;
			delete ptr;
			ptr = NULL;
			return false;
		}

		pParent = p;
		if( e > p->e)
		{
			p = p->rightChild;
		}
		else
		{
			p = p->leftChild;
		}
	}
	
	if( e < pParent->e )
	{
		pParent->leftChild = ptr;
	}
	if( e > pParent->e)
	{
		pParent->rightChild = ptr;
	}

	m_Length++;
	return true;
}

template<typename Type> bool BSTrees<Type>::erase1(Type e)
{
	Node *p = find(e);
	if ( p == NULL )
	{
		cout<<"There's no this element to delete"<<endl;
		return false;
	}

	if ( p->leftChild == NULL)
	{
		Node* pParent = getParent(p);
		if( pParent->leftChild == p )
			pParent->leftChild = p->rightChild;
		else
			pParent->rightChild = p->rightChild;
		delete p;
		p = NULL;

		m_Length--;
		return true;
	}

	if ( p->rightChild == NULL)
	{
		Node* pParent = getParent(p);
		if( pParent->leftChild == p)
			pParent->leftChild = p->leftChild;
		else
			pParent->rightChild = p->leftChild;
		delete p;
		p = NULL;

		m_Length--;
		return true;	
	}

	if ( p->leftChild != NULL && p->rightChild != NULL)
	{
		Node* pParent = getParent(p);
		Node* pPre = p->leftChild;
		Node* ptmp = pPre;
		while( pPre->rightChild != NULL )
		{
			ptmp = pPre;
			pPre = pPre->rightChild;
		}
		if( ptmp != pPre)
		{	ptmp->rightChild = pPre->leftChild;
			pPre->leftChild = p->leftChild;
			pPre->rightChild = p->rightChild;
		}
		else
			pPre->rightChild = p->rightChild;

		if( pParent == NULL )
			m_root = pPre;
		else 
			if( pParent->leftChild == p)  
				pParent->leftChild = pPre;
			else
				pParent->rightChild = pPre;
		
		delete p;
		p = NULL;

		m_Length--;
		return true;
	}
	
}

template<typename Type> bool	BSTrees<Type>::erase2(Type e)
{
	Node *p = find(e);
	if ( p == NULL )
	{
		cout<<"There's no this element to delete"<<endl;
		return false;
	}

	if ( p->leftChild == NULL)
	{
		Node* pParent = getParent(p);
		if( pParent->leftChild == p )
			pParent->leftChild = p->rightChild;
		else
			pParent->rightChild = p->rightChild;
		delete p;
		p = NULL;

		m_Length--;
		return true;
	}

	if ( p->rightChild == NULL)
	{
		Node* pParent = getParent(p);
		if( pParent->leftChild == p)
			pParent->leftChild = p->leftChild;
		else
			pParent->rightChild = p->leftChild;
		delete p;
		p = NULL;

		m_Length--;
		return true;	
	}
	if( p->leftChild != NULL && p->rightChild != NULL)
	{
		Node* pParent = getParent(p);
		Node* ptr = p->leftChild;
		while( ptr->rightChild != NULL )
			ptr = ptr->rightChild;
		ptr->rightChild = p->rightChild;

		if( pParent == NULL )
			m_root = p->leftChild;
		else
			if(pParent->leftChild == p)
				pParent->leftChild = p->leftChild;
			else
				pParent->rightChild = p->leftChild;
		delete p;
		p = NULL;
		m_Length--;
		return true;
	}
}

template<typename Type> void	BSTrees<Type>::clear()
{
	if( m_root == NULL )
		return;
	clears(m_root);
	m_root = NULL;
}

template<typename Type> void	 BSTrees<Type>::clears(Node* &p)
{
	if(p->leftChild != NULL)
	{
		clears(p->leftChild);
		p->leftChild = NULL;
	}
	if(p->rightChild != NULL)
	{
		clears(p->rightChild);
		p->rightChild = NULL;
	}
	delete p;
	p = NULL;
	m_Length--;
	
}
template<typename Type> typename BSTrees<Type>::Node* BSTrees<Type>::getRoot()
{
	return m_root;
}
template<typename Type> int		BSTrees<Type>::getLength()
{
	return m_Length;
}
template<typename Type> typename BSTrees<Type>::Node* BSTrees<Type>::getParent(Node* p)
{
	if( p == m_root)
		return NULL;
	Node* ptr = m_root;
	Node* ptf = ptr;
	while( ptr != NULL )
	{
		if ( ptr->e == p->e )
			return ptf;
		if ( ptr->e > p->e )
		{
			ptf = ptr;
			ptr = ptr->leftChild;
		}
		else
		{
			ptf = ptr;
			ptr = ptr->rightChild;
		}
	}

}

template<typename Type> typename BSTrees<Type>::Node* BSTrees<Type>::find(Type e)
{
	Node* ptr = m_root;

	while(ptr != NULL)
	{
		if ( ptr->e == e )
			return ptr;
		if ( ptr->e > e )
			ptr = ptr->leftChild;
		else
			ptr = ptr->rightChild;
	}
	//if ( ptr == NULL )
	return NULL;
}

template<typename Type> void BSTrees<Type>::traverse(Node* ptr)
{
	if( ptr == NULL )
		return;
	if( ptr->leftChild != NULL )
		traverse(ptr->leftChild);
	cout<<"Element value:"<<ptr->e<<endl;
	if( ptr->rightChild != NULL )
		traverse(ptr->rightChild);
}

template<typename Type> void BSTrees<Type>::traverseLevel(Node* root)
{
	if( root == NULL)
	{
		cout<<"This is a empty tree"<<endl;
		return;
	}
	queue<Node*> que;
	que.push(root);
	while( !que.empty() )
	{
		Node* ptr = que.front();
		que.pop();
		cout<<ptr->e<<endl;
		if(ptr->leftChild != NULL)
			que.push(ptr->leftChild);
		if(ptr->rightChild != NULL)
			que.push(ptr->rightChild);
	}
}
template<typename Type> BSTrees<Type>::~BSTrees()
{
	clear();
}
#endif

#include <iostream>
#include "BST.h"
using namespace std;


void main()
{
	BSTrees<int>Bst;  
    int Arr[9] = {6,2,8,4,10,0,12,16,14};  
    for (int i=0;i<9;i++)  
            Bst.insert(Arr[i]);  
    Bst.traverse(Bst.getRoot());  
    cout<<"Tree'slength is:"<<Bst.getLength()<<endl; 
	Bst.traverseLevel(Bst.getRoot());
}

二叉树的层次遍历