void build(List<Integer> result, int pre, TreeNode node) {
		if (node.left == null && node.right == null) {
			result.add(pre * 10 + node.val);
			return;
		} else {

			if (node.left != null) {
				build(result, pre * 10 + node.val, node.left);
			}
			if (node.right != null) {
				build(result, pre * 10 + node.val, node.right);
			}
		}
	}