Input: [,null,,]

    \

    /

Output: [,,]

 vector<int> preorderTraversal(TreeNode* root) {

         vector<int> res={};

         if(!root)

             return res;

         stack<TreeNode*> s;

         TreeNode* cur=root;

         while(!s.empty()||cur){

             while(cur){

                 res.push_back(cur->val);

                 s.push(cur);

                 cur=cur->left;

             }

             cur=s.top();

             s.pop();

             cur=cur->right;

         }

         return res;

     }

 void preOrder(TreeNode* root,vector<int>& m){

         if(!root)

             return;

         m.push_back(root->val);

         preOrder(root->left,m);

         preOrder(root->right,m);

     }

     vector<int> preorderTraversal(TreeNode* root) {

         vector<int> res={};

         if(!root)

             return res;

         preOrder(root,res);

         return res;

     }