LeetCode——Binary Tree Preorder Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    public  List<Integer> list;
    
    public List<Integer> preorderTraversal(TreeNode root) {
        
         list = new ArrayList<Integer>();
         if(root==null) {
             return list;
         }
        
        pretraversal(root);
        return list;
        
    }
    
    public void pretraversal(TreeNode node) {
        if(node == null) return;
        list.add(node.val);
        pretraversal(node.left);
        pretraversal(node.right);
    } 
}