LeeCode Binary Tree Preorder ,inorder ,postorder Traversal

class TreeNode {
		private int val;
		TreeNode left;
		TreeNode right;
		public TreeNode(int val) {
			this.val = val;
			this.left = null;
			this.right = null;
		}
	}
	
	private ArrayList<Integer> l = new ArrayList<Integer>();
	
	
	 public void preorderTraversal(TreeNode root) {
		 if(root!=null){
		 l.add(root.val);
		 }else{
			 return ;
		 }
		 preorderTraversal(root.left);
		 preorderTraversal(root.right);
		 return ;
	 }