給定一個二叉樹，返回它的 前序 遍歷。

 示例:

輸入: [1,null,2,3]  
   1
    \
     2
    /
   3 

輸出: [1,2,3]

/**
 * 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> preorderTraversal(TreeNode root) {
        List<Integer> resultList = new ArrayList<>();
        if(root == null){
            return resultList;
        }
        
        helper(resultList,root);
    
        return resultList;
    }
    
    public void helper(List<Integer> resultList,TreeNode root){
        if(root==null)
            return;
        
        resultList.add(root.val);
        helper(resultList,root.left);
        helper(resultList,root.right);
        
        
    }
    
    
}