[LeetCode] Binary Tree Preorder Traversal

Question:

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 (C++):

/** * 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) {        // IMPORTANT: Please reset any member data you declared, as        // the same Solution instance will be reused for each test case.        vector<int> result;        if (root == NULL) return result;                stack<TreeNode*> sta;        TreeNode *temp = NULL;        sta.push(root);                while (!sta.empty())        {            temp = sta.top();            result.push_back(temp -> val);            sta.pop();            if (temp -> right != NULL)            {                sta.push(temp -> right);            }            if(temp -> left != NULL)            {                sta.push(temp -> left);            }        }        return result;    }};