# Tree Traversal

Tree traversal algorithms: pre_order, in_order, post_order

void preorder_traversal_iteratively(TreeNode* root)
{
if (root == 0)
return;
stack<TreeNode*> s;
s.push(root);
cout << root->val << ' '; // visit root
TreeNode* last_pop = root;
while (!s.empty())
{
TreeNode* top = s.top();
if (top->left != 0 && top->left != last_pop && top->right != last_pop) // push_left
{
s.push(top->left);
cout << top->left->val << ' '; // visit top->left
}
else if (top->right != 0 && top->right != last_pop && (top->left == 0 || top->left == last_pop)) // push_right
{
s.push(top->right);
cout << top->right->val << ' '; // visit top->right
}
else // pop
{
s.pop();
last_pop = top;
}
}
}

void inorder_traversal_iteratively(TreeNode* root)
{
if (root == 0)
return;
stack<TreeNode*> s;
s.push(root);
TreeNode* last_pop = root;
while (!s.empty())
{
TreeNode* top = s.top();
if (top->left != 0 && top->left != last_pop && top->right != last_pop) // push_left
{
s.push(top->left);
}
else if (top->right != 0 && top->right != last_pop && (top->left == 0 || top->left == last_pop)) // push_right
{
s.push(top->right);
cout << top->val << ' '; // visit top
}
else // pop
{
s.pop();
last_pop = top;
if (top->right == 0)
cout << top->val << ' '; // visit top
}
}
}

void postorder_traversal_iteratively(TreeNode* root)
{
if (root == 0)
return;
stack<TreeNode*> s;
s.push(root);
TreeNode* last_pop = root;
while (!s.empty())
{
TreeNode* top = s.top();
if (top->left != 0 && top->left != last_pop && top->right != last_pop) // push_left
{
s.push(top->left);
}
else if (top->right != 0 && top->right != last_pop && (top->left == 0 || top->left == last_pop)) // push_right
{
s.push(top->right);
}
else // pop
{
s.pop();
last_pop = top;
cout << top->val << ' '; // visit top
}
}
}