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?

Solution:

Code:

<span style="font-size:14px;">/** * 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<int> preorderTraversal(TreeNode *root) {        vector<int> result;        if (root == NULL) return result;        stack<TreeNode *> stk;        stk.push(root);        while (!stk.empty()) {            root = stk.top();            stk.pop();            result.push_back(root->val);            if (root->right) stk.push(root->right);            if (root->left) stk.push(root->left);        }        return result;    }};</span>