Binary Tree Paths

public static List<String> binaryTreePaths( TreeNode root ) {
    List<String> res = new ArrayList<>();
    if( root == null ) return res;
    helper( root, root.val+"", res );
    return res;
}

private static void helper( TreeNode root, String tmpRes, List<String> res ) {
    if( root.left == null && root.right == null ) {
        res.add( tmpRes );
        return;
    }
    if( root.left != null )
        helper( root.left, tmpRes+"->"+root.left.val, res );
    if( root.right != null )
        helper( root.right, tmpRes+"->"+root.right.val, res );
}

public static List<String> binaryTreePathsIterative( TreeNode root ) {
    List<String> res = new ArrayList<>();
    if( root == null ) return res;

    Stack<TreeNode> stack = new Stack<>();
    Stack<String> tmpRes = new Stack<>();
    stack.push( root );
    tmpRes.push( Integer.toString( root.val ) );
    while( !stack.isEmpty() ) {
        TreeNode node = stack.pop();
        String tmpStr = tmpRes.pop();
        if( node.left == null && node.right == null ) {
            res.add( tmpStr );
            continue;
        }
        if( node.right != null ) {
            stack.push( node.right );
            tmpRes.push( tmpStr + "->" + node.right.val );
        }
        if( node.left != null ) {
            stack.push( node.left );
            tmpRes.push( tmpStr + "->" + node.left.val );
        }
    }
    return res;
}