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?

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector
     
       preorderTraversal(TreeNode *root) {        vector
      
        res;        if(root==NULL)return res;        stack
       
         sta;        sta.push(root);        while(!sta.empty())        {            TreeNode *tmp=sta.top();            sta.pop();            res.push_back(tmp->val);            if(tmp->right)sta.push(tmp->right);            if(tmp->left)sta.push(tmp->left);        }        return res;    }/**void preorder(vector
        
          &res, TreeNode * root){    if(root==NULL)return;    res.push_back(root->val);    preorder(res,root->left);    preorder(res,root->right);}    vector
         
           preorderTraversal(TreeNode *root) {        vector
          
            res; preorder(res,root); return res; }*/};