leetcode:Binary Tree Preorder Traversal

class Solution {
public:
    vector
  
    preorderTraversal(TreeNode *root) {
        vector
   
     res; stack
    
      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; } };