树的非递规遍历

#include <iostream>
#include <stack>

using namespace std;

struct Node{
	int val;
	Node* left;
	Node* right;
	Node(int x_) : val(x_), left(NULL), right(NULL){}
};

void InOrder(Node* root);
void PostOrder(Node* root);
void PreOrder(Node* root);

void CreateTree(Node** root)
{
	int x;
	Node *p;
	if (*root == NULL){
		cout << "plz input the root value(can't be 0): ";
		cin >> x;
		p = new Node(x);
		(*root) = p;
	}

	cout << "input " << (*root)->val << "'s lchild(0 to end): ";
	if (cin >> x && x != 0){
		p = new Node(x);
		(*root)->left = p;
	}
	cout << "input " << (*root)->val << "'s rchild(0 to end): ";
	if (cin >> x && x != 0){
		p = new Node(x);
		(*root)->right = p;
	}
	if ((*root)->left)
		CreateTree(&((*root)->left));
	if ((*root)->right)
		CreateTree(&((*root)->right));
}


int main()
{
	Node *root = NULL;
	CreateTree(&root);
	cout << "InOrder..." << endl;
	InOrder(root);
	cout << "
PreOrder..." << endl;
	PreOrder(root);
	cout << "
PostOrder..." << endl;
	PostOrder(root);
	return 0;
}


void InOrder(Node *root)
{//为嘛我觉得这个很不好理解呢?
	if (root == NULL) return;

	stack<Node*> s;
	Node *p = root;

	while (p != NULL || !s.empty()){
		while (p != NULL){
			s.push(p);
			p = p->left;
		}
		if (!s.empty()){
			p = s.top();
			cout << p->val << " ";
			s.pop();
			p = p->right;
		}
	}
		
}

void PostOrder(Node* root)
{
	Node *p = root;
	stack<Node*> s;
	Node *visited;
	
	s.push(p);
	while (!s.empty()){
		p = s.top();
		if ((p->left == NULL && p->right == NULL) || p->left == visited || p->right == visited){
			cout << p->val << " ";
			visited = p;
			s.pop();
		}else {
			if (p->right && p->right != visited)
				s.push(p->right);
			if (p->left && p->left != visited)
				s.push(p->left);
		}
	}
}

void PreOrder(Node* root)
{
	if (root == NULL) return;
	stack<Node*> s;
	Node* p = root;

	while (p != NULL){
		cout << p->val << " ";
		if (p->right) s.push(p->right);
		if (p->left)
			p = p->left;
		else if (!s.empty()){
			p = s.top();
			s.pop();
		}else 
			return;
	}
}