text: class Solution:
      def largeGroupPositions(self, S: str) -> List[List[int]]:
          """Two pointers.
  
          Running time: O(n) where n is the length of S.
          """
          l, r = 0, 0
          res = []
          while r < len(s):
              while r < len(s) and s[l] == s[r]:
                  r += 1
              if r - l > 2:
                  res.append([l, r - 1])
              l = r
          return res
          