private void helper(TreeNode root, int parentVal, int[] sum) {
		
		if(root == null) return;
		
		int curtVal = parentVal*10 + root.val;
		if(root.left == null && root.right == null) {
			sum[0] += curtVal;
			return;
		}
		
		helper(root.left, curtVal, sum);
		helper(root.right, curtVal, sum);
	}