Given an array containing 0s, 1s and 2s, sort the array in-place. You should treat numbers of the array as objects, hence, we can’t count 0s, 1s, and 2s to recreate the array.

dutch_flag_sort.py

def dutch_flag_sort(arr):
  n = len(arr)
  frontPointer = 0
  backPointer = n - 1
  solutionIndex = 0

  while solutionIndex <= backPointer:
    # Swap the 0s
    if arr[solutionIndex] == 0:
      swap(solutionIndex, frontPointer, arr)
      frontPointer +=1
      solutionIndex +=1
    # Go over 1s
    elif arr[solutionIndex] == 1:
      solutionIndex +=1
    else:
      swap(solutionIndex, backPointer, arr)
      backPointer -=1

def swap(f_pointer, b_pointer, arr):
  temp = arr[b_pointer]
  arr[b_pointer] = arr[f_pointer]
  arr[f_pointer] = temp

  return 

def main():
  arr = [1, 0, 2, 1, 0]
  dutch_flag_sort(arr)
  print(arr)

  arr = [2, 2, 0, 1, 2, 0]
  dutch_flag_sort(arr)
  print(arr)


main()