#define NULL 0class Solution {public:    vector<int> preorderTraversal(TreeNode *root) {        stack<TreeNode*> s;        vector<int> v1;        if(root!=NULL)        s.push(root);        while(s.size())        {            TreeNode* tmp;            tmp = s.top();            s.pop();            v1.push_back(tmp->val);            if(tmp->right != NULL )            {                s.push(tmp->right);            }            if(tmp->left != NULL)            {                s.push(tmp->left);            }        }        return v1;    }};