input: Write a Python function to perform merge sort on an array.
  assert mergeSort([12, 11, 13, 5, 6, 7]) == [5, 6, 7, 11, 12, 13]
  assert mergeSort([4, 2, 8, 0, 5, 7, 1, 3, 9]) == [0, 1, 2, 3, 4, 5, 7, 8, 9]
  assert mergeSort([10, 80, 30, 90, 40, 50, 70]) == [10, 30, 40, 50, 70, 80, 90]
output: def mergeSort(arr):
      if len(arr) > 1:
          mid = len(arr) // 2
          L = arr[:mid]
          R = arr[mid:]
  
          mergeSort(L)
          mergeSort(R)
  
          i = j = k = 0
          
          while i < len(L) and j < len(R):
              if L[i] < R[j]:
                  arr[k] = L[i]
                  i += 1
              else:
                  arr[k] = R[j]
                  j += 1
              k += 1
  
          while i < len(L):
              arr[k] = L[i]
              i += 1
              k += 1
  
          while j < len(R):
              arr[k] = R[j]
              j += 1
              k += 1
      return arr