Algorithm, Binary tree searching, Iterative preorder traversal

iterative_preorder_traversal.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> out;
        vector<TreeNode*> nodes;
        TreeNode* node = root;
        
        while(1)
        {
            if (node != NULL) out.push_back(node->val);
            else                return out;
            
            if (node->right != NULL) nodes.push_back(node->right);
            if (node->left != NULL) nodes.push_back(node->left);
            
            if (nodes.empty())  return out;
            else 
            {
                node = nodes.back();
                nodes.pop_back();
            }
        }
        
        return out;
    }
};