Problem Challenge 2: Scheduling Tasks (hard)

from heapq import *
from collections import deque, Counter

class Solution:
  def reorganizeString(self, str, k):
    counts = Counter(str)
    heap = [
      (-value, char) for char, value in counts.items()
    ]
    heapify(heap)

    if -heap[0][0] > (len(str) + k - 1)//k:
      return ''
    
    res = list('_' * len(str))
    i = 0
    offset = 0
    while heap:
      count, char = heappop(heap)
      while count < 0:
        res[i] = char
        i += k
        if i >= len(res):
          offset += 1
          i = offset
        count += 1
    return ''.join(res)

• str consists of only lowercase English letters. - This is not correct as there are uppercase letters also.