Longest substring without any repeating characters

non_repeat_substring.py

def non_repeat_substring(string):
  from collections import Counter
  max_ctr = Counter()
  left_idx = 0
  distinct = 0
  max_distinct = float('-inf')
  for right_idx, r_char in enumerate(string):
    if not max_ctr[r_char]:
      distinct += 1
    max_ctr[r_char] += 1
    while max_ctr[r_char] > 1:  
      l_char = string[left_idx]
      max_ctr[l_char] -= 1
      if not max_ctr[l_char]:
        distinct -= 1
      left_idx += 1
    max_distinct = max(max_distinct, distinct)
  return max_distinct