830. Positions of Large Groups（easy） 

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]]

1 straightforward and awkard me

陣列類easy刷了這麼好幾道之後感覺這個型別的解法碰到不下三遍了，夠disappointed的了因為在邊界判斷上每次都要思考個幾分鐘 。

思路依舊2個標記指向一段連續子串的start和end點

class Solution:
    def largeGroupPositions(self, S):
        """
        :type S: str
        :rtype: List[List[int]]
        """
        if len(S) < 3:return []
        index = []
        start = 0
        end = 1
        while(end < len(S)):
            while(end < len(S) and S[end] == S[start]):end += 1    
            if((end - start) >= 3): index.append([start,end-1])
            start = end 
            end += 1
        return index

Runtime: 52 ms, faster than 88.11% of Python3