二叉树的三种遍历的递归实现都很简单，但是在面试中，面试官一般都不会问你递归的实现，所以学习二叉树的非递归实现还是很重要的。

#include <iostream>

using namespace std;

//Author: 过往记忆
//Blog: www.iteblog.com
//Email: wyphao.2007@163.com
///////////////////////////////////////////////////////////////////////
//stack 
template <class T>
class Stack{
public:
	Stack(int size = 50);
	~Stack();
	void push(T* data);
	T* pop();
	bool isEmpty();
	T* peek();
private:
	int size;
	int top;
	bool isFull();
	T **data;
};

template <class T>
Stack<T>::Stack(int size){
	if(size <= 0){
		cout << "分配的内存太小了" << endl; 
	}
	
	data = new T*[size];
	top = -1;
	this->size = size; 
}

template <class T>
Stack<T>::~Stack(){
	delete []data;
}

template <class T>
void Stack<T>::push(T* data){
	++top;
	if(isFull()){
		cout << "貌似栈满了耶" << endl;
		exit(1); 
	} 
	this->data[top] = data; 
} 

template <class T>
T* Stack<T>::pop(){ 
	if(isEmpty()){
		cout << "栈为空，不可以再出元素了!" << endl;
		exit(1); 
	}
	
	return data[top--]; 
}

template <class T>
T* Stack<T>::peek(){
	if(isEmpty()){
		cout << "栈为空" << endl;
		exit(1); 
	}
	
	return data[top];
}

template <class T>
bool Stack<T>::isFull(){
	if(top == size){
		return true; 
	} 
	
	return false; 
} 

template <class T>
bool Stack<T>::isEmpty(){
	if(top < 0){
		return true; 
	} 
	
	return false; 
} 
///////////////////////////////////////////////////////////////////////
//tree
template <class T> 
class BTree{
public:
	BTree *left;
	BTree *right;
	T data; 
	
	BTree() : left(NULL), right(NULL), data(NULL){};
	~BTree(){}; 
}; 

///////////////////////////////////////////////////////////////////////
template <class T> 
void PreOrder(BTree<T> *root){
	if(root != NULL){
		Stack<BTree<T> > stack ;
		BTree<T> *ptr = root;
		BTree<T> *temp; 
		stack.push(ptr);
		while(!stack.isEmpty())	{
			temp =  stack.pop(); 
			cout << temp->data << " ";
			if(temp->right != NULL){
				stack.push(temp->right);
			}
			
			if(temp->left != NULL){
				stack.push(temp->left);
			}
		}
		cout << endl; 
	} 
} 

///////////////////////////////////////////////////////////////////////
template <class T>
void InOrder(BTree<T> *root){
	if(root != NULL){
		Stack<BTree<T> > stack ;
		BTree<T> *ptr = root;
		while(!stack.isEmpty() || ptr != NULL){
			while(ptr != NULL){
				stack.push(ptr);
				ptr = ptr->left;
			}
			
			if(!stack.isEmpty()){
				ptr = stack.pop();
				cout << ptr->data << " ";
				ptr = ptr->right;
			}
			
		}
		cout << endl; 
	}
}

///////////////////////////////////////////////////////////////////////
template <class T>
void PostOrder(BTree<T> *root){
	if(root != NULL){
		Stack<BTree<T> > stack;
		BTree<T> *ptr = root;
		BTree<T> *temp;
		bool flags;
		
		do{
			while(ptr != NULL){
				stack.push(ptr);
				ptr = ptr->left;
			}
			
			temp = NULL;
			flags = true;
			
			while(flags && !stack.isEmpty()){
				ptr = stack.peek();
				if(ptr->right == temp){
					cout << ptr->data << " ";
					stack.pop();
					temp = ptr;
				}else{
					ptr = ptr->right;
					flags = false;
				}
			}
		}while(!stack.isEmpty());
		cout << endl;		
	}
}
///////////////////////////////////////////////////////////////////////
template <class T>
void PreOrder1(BTree<T> * root){
	if(root != NULL){
		cout << root->data << " ";
		PreOrder1(root->left);
		PreOrder1(root->right);
	}	
}

///////////////////////////////////////////////////////////////////////
template <class T>
void InOrder1(BTree<T> * root){
	if(root != NULL){
		InOrder1(root->left);
		cout << root->data << " ";
		InOrder1(root->right);
	}	
}

///////////////////////////////////////////////////////////////////////
template <class T>
void PostOrder1(BTree<T> * root){
	if(root != NULL){
		PostOrder1(root->left);
		PostOrder1(root->right);
		cout << root->data << " ";
	}	
}

///////////////////////////////////////////////////////////////////////
int main(){
	
	BTree<int> *root = new BTree<int>;
 	BTree<int> *A, *B, *C, *D, *E;
 	A = new BTree<int>; 
 	B = new BTree<int>; 
 	C = new BTree<int>; 
 	D = new BTree<int>; 
 	E = new BTree<int>; 
 	
 	A->data = 5; 
 	B->data = 6;
 	C->data = 4;
 	D->data = 2;
 	E->data = 7;
 	
 	root = A;
	A->left = B;
	A->right = E;
	B->left = C;
	B->right = D; 
 	/*C->left = NULL;
 	C->right = NULL;
 	D->left = NULL;
 	D->right = NULL;
 	E->left = NULL;
 	E->right = NULL;*/
 	
	cout << "非递归: " << endl;
 	PreOrder(root); 
 	InOrder(root);
 	PostOrder(root);
 	
	cout << "递归: " << endl; 	
 	PreOrder1(root);
 	cout << endl;
 	InOrder1(root);
	cout << endl;
 	PostOrder1(root);
	cout << endl;
	return 0;
}