class Solution

{

public:

    vector<int> preorderTraversal(TreeNode* root)

    {

        vector<int> v;

        if(root == NULL) return v;

        stack<TreeNode *> s;

        s.push(root);

        TreeNode *curNode = NULL;

        while(!s.empty())

        {

            curNode = s.top(); s.pop();

            v.push_back(curNode->val);

            // 注意这里的顺序

            if(curNode->right) s.push(curNode->right);

            if(curNode->left) s.push(curNode->left);

        }

        return v;

    }

};