## DEV Community

Abhishek Chaudhary

Posted on

# 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
``````