// Data Set

 // One top level comment

 var comments = [{

     id: 1,

     parent_id: null

 }, {

     id: 2,

     parent_id: 1

 }, {

     id: 3,

     parent_id: 1

 }, {

     id: 4,

     parent_id: 2

 }, {

     id: 5,

     parent_id: 4

 }];

 const nest = (items, id = null, link = 'parent_id') =>

   items

     .filter(item => item[link] === id)

     .map(item => ({ ...item, children: nest(items, item.id) }));

 nest(comments);

// divide array

function divide(p, r) {

  return Math.floor((p + r) / 2);

}

// merge array

function merge(A, p, q, r) {

  const A1 = A.slice(p, q);

  const A2 = A.slice(q, r);

  A1.push(Number.MAX_SAFE_INTEGER);

  A2.push(Number.MAX_SAFE_INTEGER);

  for (let i = p, j = 0, k = 0; i < r; i++) {

    if (A1[j] < A2[k]) {

      A[i] = A1[j];

      j++;

    } else {

      A[i] = A2[k];

      k++;

    }

  }

}

// main method

function merge_sort(A, p = 0, r) {

  r = r || A.length;

  if (r - p === 1) {

    return;

  }

  const q = divide(p, r);

  console.log(q)

  merge_sort(A, p, q);

  merge_sort(A, q, r);

  merge(A, p, q, r);

  return A;

}