(java)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].

Note: Recursive solution is trivial, could you do it iteratively?

思路:就是一个先序遍历

代码如下(已通过leetcode)

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