LeetCode——Binary Tree Preorder Traversal

Description：

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].

经典的二叉树前序遍历，用递归闭着眼写。

/**
 * 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);
    }
}