二叉树的创建，递归前序、中序、后序遍历以及队列实现层遍历

本文主要介绍二叉树的前中后序遍历的递归实现，递归实现比较简单，很多关于二叉树的实现都可以用递归，非递归实现准备用另一篇文章介绍，前序、中序、后序的遍历顺序体现的是访问根节点的顺序，前序就是先访问根节点，然后左子树，最后右子树。而中序则先访问左子树，然后访问根节点，最后访问右子树。

#include <iostream>
#include <vector>
#include <queue>
using namespace std;


struct TreeNode
{
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x),left(NULL),right(NULL){}
};

//创建二叉树
void CreateTree(TreeNode* &root)
{
	 int data;
	cin >> data;
	if(-1 == data)
	{
		root =NULL;
	}
	else
	{
		root = new TreeNode(data);
		CreateTree(root->left);
		CreateTree(root->right);
	}
}
//递归前序
void PreOrder(TreeNode *root)
{
	if(root)
	{
		cout << root->val;
		if(root->left) PreOrder(root->left);
		if(root->right) PreOrder(root->right);

	}
}
//递归中序
void MidOrder(TreeNode *root)
{
	if(root)
	{
		
		if(root->left) MidOrder(root->left);
		cout << root->val;
		if(root->right) MidOrder(root->right);

	}
}

//递归后序
void PostOrder(TreeNode *root)
{
	if(root)
	{

		if(root->left) PostOrder(root->left);		
		if(root->right) PostOrder(root->right);
		cout << root->val;

	}
}

//队列实现层遍历
void levelOrder(TreeNode *root)
{
	if(NULL == root) return;
	queue <TreeNode *> que;
	que.push(root);
	while(!que.empty())
	{
		TreeNode *node = que.front();
		cout << node->val;
		que.pop();
		if(node->left)
		{
			que.push(node->left);
		}
		if(node->right)
		{
			que.push(node->right);
		}
	}
		
}

int main()
{
	
	TreeNode *root =NULL;
	CreateTree(root);
	cout<<endl;
	PreOrder(root);
	cout<<endl;
	MidOrder(root);
	cout<<endl;
	PostOrder(root);
	cout<<endl;
	levelOrder(root);
	getchar();
}