Given a binary tree, return the preorder traversal of its nodes' values.

For example:

Given binary tree {1,#,2,3},

1    \     2    /   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

class Solution {public:    vector
     
       preorderTraversal(TreeNode *root) {        vector
      
        result;        if(!root) return result;                stack
       
         treeStack;        treeStack.push(root);        TreeNode* current;        while(!treeStack.empty())        {            current = treeStack.top();            treeStack.pop();            result.push_back(current->val);            if(current->right) treeStack.push(current->right);            if(current->left) treeStack.push(current->left);        }        return result;    }};