class Solution:
    def largeGroupPositions(self, S):
        """
        :type S: str
        :rtype: List[List[int]]
        """

        last = None
        result = []
        start = 0
        end = 0

        for i in range(len(S)):
            if S[i] == last:
                end = i
            else:
                if end - start >= 2:
                    result.append([start, end])

                last = S[i]
                start = i
                end = i

        if end - start >= 2:
                    result.append([start, end])

        return result