leetcode:Binary Tree Preorder Traversal

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