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".
A group is identified by an interval [start, end], where start and end denote the start and end indices (inclusive) of the group. In the above example, "xxxx" has the interval [3,6].
A group is considered large if it has 3 or more characters.
Return the intervals of every large group sorted in increasing order by start index.
Example 1:
Input: s = "abbxxxxzzy"
Output: [[3,6]]
Explanation: "xxxx" is the only large group with start index 3 and end index 6.
Example 2:
Input: s = "abc" Output: [] Explanation: We have groups "a", "b", and "c", none of which are large groups.
Example 3:
Input: s = "abcdddeeeeaabbbcd" Output: [[3,5],[6,9],[12,14]] Explanation: The large groups are "ddd", "eeee", and "bbb".
Example 4:
Input: s = "aba" Output: []
Constraints:
1 <= s.length <= 1000-
scontains lower-case English letters only.
较大分组的位置。
在一个由小写字母构成的字符串 s 中，包含由一些连续的相同字符所构成的分组。
例如，在字符串 s = "abbxxxxzyy" 中，就含有 "a", "bb", "xxxx", "z" 和 "yy" 这样的一些分组。
分组可以用区间 [start, end] 表示，其中 start 和 end 分别表示该分组的起始和终止位置的下标。上例中的 "xxxx" 分组用区间表示为 [3,6] 。
我们称所有包含大于或等于三个连续字符的分组为 较大分组 。
找到每一个 较大分组 的区间，按起始位置下标递增顺序排序后，返回结果。
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/positions-of-large-groups
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这道题的题意简化一下就是找input字符串中长度大于等于3的子串，子串里面都是相同字母，输出这些子串的[start, end]。思路是two pointer。注意当两个指针指向的字母不同的时候，要立即将左指针放到右指针的位置上，再进行下面的扫描。
时间O(n)
空间O(n) - output
Java实现
1 class Solution {
2 public List<List<Integer>> largeGroupPositions(String s) {
3 List<List<Integer>> res = new ArrayList<>();
4 int i = 0;
5 for (int j = 1; j < s.length(); j++) {
6 while (j < s.length() && s.charAt(i) == s.charAt(j)) {
7 j++;
8 }
9 if (j - i >= 3) {
10 res.add(Arrays.asList(i, j - 1));
11 }
12 i = j;
13 }
14 return res;
15 }
16 }