function mergeSort (arr) {
  function merge (l, m, r) {
    let i = 0, j = 0, k = l

    const leftL = m - l + 1
    const rightL = r - m

    const tempL = []
    const tempR = []

    for (; i < leftL; i++)
      tempL[i] = arr[l + i]

    for(; j < rightL; j++)
      tempR[j] = arr[m + 1 + j]

    i = j = 0

    while (i < leftL && j < rightL) {
      if (tempL[i] < tempR[j])
        arr[k++] = tempL[i++]
      else
        arr[k++] = tempR[j++]
    }

    while (i < leftL)
      arr[k++] = tempL[i++]

    while (j < rightL)
      arr[k++] = tempR[j++]
  }

  function sort (l, r) {
    if (l < r) {
      const m = Math.floor((r + l) / 2)

      sort(l, m)
      sort(m + 1, r)

      merge(l, m, r)
    }
  }

  sort(0, arr.length - 1)
}