二叉树中序遍历的非递归方法

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