[LeetCode]:144：Binary Tree Preorder Traversal

题目：

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [1,2,3].

代码：

public class Solution {
    public static ArrayList<Integer> listResult = new ArrayList<Integer>();
    public List<Integer> preorderTraversal(TreeNode root) {
        listResult.clear();
        if(root!=null){
            getNode(root);
        }
        return listResult;
    }
    public static void getNode(TreeNode root){
        if(root == null){
           return;
        }
        listResult.add(root.val);
        getNode(root.left);
        
        getNode(root.right);
    }
}