144. Binary Tree Preorder Traversal

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].

代码如下：

 /**
  * 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> preorderTraversal(TreeNode root) {
         List<Integer> list=new ArrayList<>();
         if(root==null)
         return list;

         list.add(root.val);

         if(root.left!=null)
         list.addAll(preorderTraversal(root.left));
         if(root.right!=null)
         list.addAll(preorderTraversal(root.right));

         return list;
     }
 }