Binary Tree Pre-order Traversal

Given a binary tree, traverse the tree in pre-order, print nodes' values.

Preorder traversal:  

1. Visit the root.
2. Traverse the left subtree.
3. Traverse the right subtree.

/**

 * Definition of a binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

A recursive solution is simple: 

void pre-order( TreeNode *root)

{

    if (not root)

        return ;

     std:cout<< root->val << ' ';

     pre-order(root->left);

     pre-order(root->right);

}

What about an iterative solution? 

Hint: use stack.