一個BST， 問怎麽找到第二大的節點。首先說了個naive的方法，serialize, 他問有沒有更有效的方法。最後用的iterative的方法向右遍歷子節點 log(n)

或者inorder, O(n)：

 public int find(TreeNode root) {
        TreeNode first = root;
        TreeNode second = null;
        
        while (root != null) {
            if (root.right == null && root.left == null) {
              return second.val;
            } else if (root.right != null) {
              second = first;
              first = root.right;
              root = root.right;
            } else if (root.left != null) {
              
              root = root.left;
              break;
            }
        }
        while (root != null) {
          if (root.right == null) {
            return root.val;
          } else {
            root = root.right;
          }
        }
        return -1;
    }

find second maximum element in Binary search tree