if (0 === arr.length) {

    console.log('input array is empty!');

} else {

    var arr2 = arr.slice();

    arr2.sort();

    if (1 <= arr2.length) {

        arr2[0] = '欢迎' + arr2[0];

        arr2[arr2.length-1] = arr2[arr2.length-1] + '同学！';

    }

    if (2 <= arr2.length) {

       arr2[arr2.length-1] = '和' + arr2[arr2.length-1];

       arr2[arr2.length-2] = arr2[arr2.length-2] + arr2[arr2.length-1];

       arr2.pop();

    }

    console.log(arr2.join('，'));

}