一、先序遍历

1、递归算法

struct Tree
{
	int date;
	Tree* lchild;
	Tree* rchild;
	Tree(int x):date(x),lchild(nullptr),rchild(nullptr){}
};

void PreOrder(Tree* root)
{
	if (root != nullptr)
	{
		cout << root->date << " ";
		PreOrder(root->lchild);
		PreOrder(root->rchild);
	}
}

2、非递归算法

void PreOrder(Tree* root)
{
	stack<Tree* > Stack;
	if (root == nullptr)
		return;
	while (root != nullptr || !Stack.empty())
	{
		while (root != nullptr)
		{
			Stack.push(root);
			cout << root->date << " ";
			root = root->lchild;
		}
		root = Stack.top();
		Stack.pop();
		root = root->rchild;
	}
}

二、中序遍历

1、递归算法

void InOrder(Tree* root)
{
	if (root != nullptr)
	{
		InOrder(root->lchild);
		cout << root->date << " ";
		InOrder(root->rchild);
	}
}

2、非递归算法

void InOrder(Tree* root)
{
	stack<Tree*> s;
	while (root != nullptr || !s.empty())
	{
		if (root != nullptr)
		{
			s.push(root);
			root = root->lchild;
		}
		else
		{
			root = s.top();s.pop();
			cout << root->date << " ";
			root = root->rchild;
		}
	}
}

三、后序遍历

1、递归算法

void PostOrder(Tree* root)
{
	if (root != nullptr)
	{
		PostOrder(root->lchild);
		PostOrder(root->rchild);
		cout << root->date << " ";
	}
}

2、非递归算法

void PostOrder(Tree* root)
{
	stack<Tree*> s;
	Tree* r = nullptr;//使用辅助指针，指向最近访问过的节点
	while (root != nullptr || !s.empty())
	{
		if (root != nullptr)
		{
			s.push(root);
			root = root->lchild;
		}
		else
		{
			root = s.top();
			if (root->rchild != nullptr && root->rchild != r)
				root = root->rchild;
			else
			{
				s.pop();
				cout << root->date << " ";
				r = root;
				root = nullptr;
			}
		}
	}
}

四、层次遍历

1、方法1

void LevelOrder(Tree* root)
{
	if (root == nullptr)
		return;
	queue<Tree*> que;
	que.push(root);
	while (!que.empty())
	{
		root = que.front();
		cout << root->date << " ";
		que.pop();
		if (root->lchild != nullptr)
			que.push(root->lchild);
		if (root->rchild != nullptr)
			que.push(root->rchild);
	}
}

2、方法2

void LevelOrder(Tree* root)
{
	if (root == nullptr)
		return;
	queue<Tree*> que;
	que.push(root);
	while (!que.empty())
	{
		int size = que.size();
		while (size)
		{
			root = que.front();
			cout << root->date << " ";
			que.pop();
			if (root->lchild != nullptr)
				que.push(root->lchild);
			if (root->rchild != nullptr)
				que.push(root->rchild);
			--size;
		}
	}
}