[LeetCode&Python] Problem 830. Positions of Large Groups

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