830. Positions of Large Groups

Problem

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 (Java)

class Solution {
    public List<List<Integer>> largeGroupPositions(String s) {
        List<List<Integer>> map = new ArrayList<>();
        int i = 0;
        while (i < s.length()) {
            int j = i;
            while (j < s.length() && s.charAt(j) == s.charAt(i)) {
                j++;
            }
            if ((j - 1) - i + 1 >= 3) {
                map.add(Arrays.asList(i, j - 1));
            }
            i = j;
        }
        return map;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).