Leetcode 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].

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

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