[LeetCode]Positions of Large Groups

题目描述：

LeetCode 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

题目大意：

给定字符串S，求其中所有连续出现次数大于3次的字母的起止位置。

解题思路：

双指针法（Two Pointers）

遍历S，前后“指针”维护连续字符的起止下标

Python代码：

class Solution(object):
    def largeGroupPositions(self, S):
        """
        :type S: str
        :rtype: List[List[int]]
        """
        j = -1
        d = ''
        ans = []
        for i, c in enumerate(S + '#'):
            if c != d:
                if i - j >= 3:
                    ans.append([j, i - 1])
                j = i
            d = c
        return ans