public List<Integer> inorderTraversal(TreeNode root) {

     List<Integer> res = new ArrayList<Integer>();

     if(root != null) {

         Stack<TreeNode> s = new Stack();

         s.push(root);

         while(!s.empty()){

             while(root != null && root.left != null){

                 s.push(root.left);

                 root = root.left;

             }

             root = s.pop();

             res.add(root.val);

             root = root.right;            

             if(root != null)

                 s.push(root);

         }

     }

 return res;

 }