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".

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".

Constraints:

• 1 <= s.length <= 1000
• s contains lowercase English letters only.

SOLUTION:

class Solution:
    def largeGroupPositions(self, s: str) -> List[List[int]]:
        n = len(s)
        i = 0
        op = []
        while i < n:
            ctr = 1
            while i < n - 1 and s[i] == s[i + 1]:
                ctr += 1
                i += 1
            i += 1
            if ctr >= 3:
                op.append([i - ctr, i - 1])
        return op