comefrom: <doocs>1180.Count Substrings with Only One Distinct Letter_1.py
code: def countLetters( s: str) -> int:
      n = len(s)
      i = ans = 0
      while i < n:
          j = i
          while j < n and s[j] == s[i]:
              j += 1
          ans += (1 + j - i) * (j - i) // 2
          i = j
      return ans
masked: def sample_funcname ( var_0 : str ) -> int :
      var_4 = len ( var_0 )
      var_7 = var_5 = 0
      while var_7 < var_4 :
          var_8 = var_7
          while var_8 < var_4 and var_0 [ var_8 ] == var_0 [ var_7 ] :
              var_8 += 1
          var_5 += ( 1 + var_8 - var_7 ) * ( var_8 - var_7 ) // 2
          var_7 = var_8
      return var_5
  