144. 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:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> result;
TreeNode *p = root;
stack<TreeNode *>s;

if (root != NULL)
s.push(root);

while (!s.empty()) {
p = s.top();
s.pop();
result.push_back(p->val);

if (p->right != NULL) s.push(p->right);
if (p->left != NULL) s.push(p->left);
}
return result;
}
};