Binary Tree Preorder Traversal

class Solution{
public:
	vector<int>preorderTraversal(TreeNode *root)
	{
		vector<int>preOrder;
		Traversal(root,preOrder);
		return preOrder;
	}
	stack<TreeNode*> st;
private:
	void Traversal(TreeNode *root,vector<int>&preOrder)
	{
		if(root==NULL)
			return;
		TreeNode *p;
		TreeNode *pre=NULL;
		bool flag=true;
		p=root;
		while(p||!st.empty())
		{
			if(p)
			{
				st.push(p);
				preOrder.push_back(p->val);
				p=p->left;
			}
			else
			{
				p=st.top();
				st.pop();
				p=p->right;
			}
		}

	}
};