前序、中序、后序遍历二叉树的非递归实现

class Node
{
public:
int data;
Node* left;
Node* right;
};

void pre-order(Node* root)
{
stack<Node*> stk;
if (root)
stk.push(root);
while (!stk.empty())
{
Node* nd = stk.top();
cout << nd->data << endl;
stk.pop();
if (nd->right)
stk.push(nd->right);
if (nd->left)
stk.push(nd->left);
}
}

void in-order(Node* root)
{
stack<Node*> stk;
Node* nd = root;
while (nd)
{
stk.push(nd);
nd = nd->left;
}
while (!stk.empty())
{
nd = stk.top();
cout << nd->data << endl;
stk.pop();
Node* nd2 = nd->right;
while (nd2)
{
stk.push(nd2);
nd2 = nd2->left;
}
}
}

void post-order(Node* root)
{
// 使用双队列
stack<Node*> stk1, stk2;
if (root)
stk1.push(root);
while (!stk1.empty())
{
Node* nd = stk1.top();
stk1.pop();
stk2.push(nd);
if (nd->left)
stk1.push(nd->left);
if (nd->right)
stk1.push(rd->right);
}
while (!stk2.empty())
{
Node* nd = stk2.top();
cout << nd->data << endl;
stk2.pop();
}
}