public int[] inorderTraversal (TreeNode root) { // write code here List<Integer> res = new ArrayList<>(); dfs(root,res); return res.stream().mapToInt(Integer::valueOf).toArray(); } private void dfs(TreeNode node,List<Integer> res){ if(node==null) return ; dfs(node.left,res); res.add(node.val); dfs(node.right,res); }