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;
}