LeetCode 94. Middle order traversal of binary tree

https://leetcode-cn.com/problems/binary-tree-inorder-traversal/
In the sequence traversal : Left root right

 public List<Integer> inorderTraversal(TreeNode root) {
    
        List<Integer> list = new ArrayList<>();
        inorder(root, list);

        return list;
    }
     public void inorder(TreeNode root, List<Integer> list) {
    
         if (root == null) {
    
             return;
         }
         inorder(root.left,list);
         list.add(root.val);
         inorder(root.right, list);
     }