LC144 Binary Tree Preorder Traversal

class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> result;
if(root==NULL)
return result;
stack<TreeNode*> is;
TreeNode* tmp=root;
while(!is.empty()||tmp!=NULL)
{
if(tmp!=NULL)
{
is.push(tmp);
result.push_back(tmp->val);
tmp=is.top()->left;
}
else if(tmp==NULL)
{
tmp=is.top()->right;
is.pop();
}
}
return result;
}
};