[Leetcode] Binary Tree Preorder Traversal

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 a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    /*algorithm: recrusive
    */
    void preorder(TreeNode* root,vector<int>&path){
        if(!root)return;
        path.push_back(root->val);
        preorder(root->left,path);
        preorder(root->right,path);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int>path;
        preorder(root,path);
        return path;
    }
};

/**
 * 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:
    /*algorithm: iterative algorithm
    */
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int>path;
        stack<TreeNode*>stk;
        if(!root)return path;
        stk.push(root);
        while(!stk.empty()){
            TreeNode* t = stk.top();stk.pop();
            path.push_back(t->val);
            if(t->right)stk.push(t->right);
            if(t->left)stk.push(t->left);
        }
        return path;
    }
};