题目地址：https://leetcode.com/problems/positions-of-large-groups/description/
题目描述
Ina string S of lowercase letters, these letters form consecutive groups of the same character.
Forexample, 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.
Thefinal 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
一个长字符串可以按照字符的连续出现，分组。每个组内都是一段连续的，字符相同的子字符串。
要求，长度不小于3的所有组的字符串起始和结束位置。
解题方法
直接暴力求解即可！从左到右遍历字符串，只要后面的字符和该组起始的字符相同，那么就是属于同一个组；否则，开辟一个新组，并且判断之前的这个组长度是否>=3，是的话进行保存。
第一遍提交没通过的原因是忘了判断，当字符串结束的时候也是一个组终止的标志。比如字符串"aaa"。
class Solution:
def largeGroupPositions(self, S):
"""
:type S: str
:rtype: List[List[int]]
"""
groups = []
before_index, before_char = 0, S[0]
for i, s in enumerate(S):
if s != before_char:
if i - before_index >= 3:
groups.append([before_index, i - 1])
before_index = i
before_char = s
if i - before_index >= 2:
groups.append([before_index, i])
return groups
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二刷的时候，对结尾的判断是添加了一个大写字符，这样的话在不打扰之前小写字符串的基础上，增加了结束符号。
class Solution:
def largeGroupPositions(self, S):
"""
:type S: str
:rtype: List[List[int]]
"""
S = S + "A"
groups = []
previndex, prevc = 0, ""
for i, c in enumerate(S):
if not prevc:
prevc = c
previndex = i
elif prevc != c:
if i - previndex >= 3:
groups.append([previndex, i - 1])
previndex = i
prevc = c
return groups
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