/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List <Integer> res=new ArrayList<>();
        helper(root,res);
        return res;
    }
    
    public void helper(TreeNode root,List<Integer> res){
        if(root!=null)//java中没有NULL只有小写
        {
            if(root.left!=null)
            {
                helper(root.left,res);
            }
            res.add(root.val);
            if(root.right!=null)
            {
                helper(root.right,res);
            }
        }
    }
}