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public void preorder(List<Integer> list, TreeNode root){
		if(root==null){
			return;
		}

		list.add(root.val);

		preorder(list, root.left);
		preorder(list, root.right);
	}
    public int[] preorderTraversal (TreeNode root) {
        
		List<Integer> list = new ArrayList<>();
		preorder(list, root);

		int[] ans = new int[list.size()];

		for(int i=0;i<list.size();i++){
			ans[i]=list.get(i);
		}

		return ans;
    }