inorder traversal
class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> list = new ArrayList<Integer>();
return dfs(root, list);
}
private List<Integer> dfs(TreeNode root, List<Integer> list)
{
if(root == null)
return list;
list = dfs(root.left, list);
list.add(root.val);
return dfs(root.right,list);
}
}