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.

给出字符串，让你找连续相同的长度大于等于3的子字符串。

class Solution(object):
    def largeGroupPositions(self, S):
        """
        :type S: str
        :rtype: List[List[int]]
        """
        l = 0
        r = 0
        n = len(S)
        ans = []
        while r < n:
            while r < n and S[l] == S[r]:
                r += 1
            length = r - l
            if length >= 3 and length:
                ans.append([l, r - 1])
            l = r
        return ans