function MergeSort(arr){
  if(arr.length <= 1){
    return arr;
  }
  helper(arr, 0, arr.length-1)
  return arr;
}

function helper(A, start, end){
  //Based Case for Leaf Node
  if(start == end){
    return;
  }

  //Case for Internal node worker
  let mid = Math.floor((start + end) / 2);
  //console.log(start, end, mid);
  helper(A, start, mid);
  helper(A, mid + 1, end);

  //merge two sorted half arrays
  let i = start;
  let j = mid + 1;

  let auxArray = new Array();

  while((i <= mid) && (j <= end)){
    // console.log(A[i], A[j]);
    if(A[i] <= A[j]){
      auxArray.push(A[i]);
      i++;
    }else if(A[i] > A[j]) {
      auxArray.push(A[j]);
      j++
    }
  }

  //Append remaining items
  while(i <= mid){
    auxArray.push(A[i]);
    i++
  }
  while(j <= end){
    auxArray.push(A[j]);
    j++;
  }

  for(let k = 0; k < auxArray.length; k++){
      A[start+k] = auxArray[k];
  }

  return;
}


let number = [1,3,7,4]
console.log(MergeSort(number))