In a string S of lowercase letters, these letters form consecutive groups of the same character.

For example, a string like S = "abbxxxxzyy" has the groups "a", "bb", "xxxx", "z" and "yy".

Call a group large if it has 3 or more characters.  We would like the starting and ending positions of every large group.

The final answer should be in lexicographic order.

Example 1:

Input: "abbxxxxzzy"

Output: [[3,6]]

Explanation: "xxxx" is the single large group with starting  3 and ending positions 6.

Example 2:

Input: "abc"

Output: []

Explanation: We have "a","b" and "c" but no large group.

Example 3:

Input: "abcdddeeeeaabbbcd"

Output: [[3,5],[6,9],[12,14]]

Note:  1 <= S.length <= 1000

class Solution(object):

    def largeGroupPositions(self, S):

        """

        :type S: str

        :rtype: List[List[int]]

        """

        start=end=-1

        count=0

        ans=[]

        for i in range(len(S)):

            if count>0:

                if S[i]!=S[i-1]:

                    if count>=3:

                        end=i-1

                        ans.append([start,end])

                        start=i

                        count=1

                    else:

                        start=i

                        count=1

                else:

                    count+=1

            else:

                count+=1

                start=i

        if count>=3:

            ans.append([start,len(S)-1])

        return ans