LeetCode 830 - Positions of Large Groups

https://leetcode.com/problems/positions-of-large-groups/description/

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".

Call a group large if it has 3 or more characters.  We would like the starting and ending positions of every large group.

The final 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]]

    public List<List<Integer>> largeGroupPositions(String S) {

        List<List<Integer>> ans = new ArrayList();

        int i = 0, N = S.length(); // i is the start of each group

        for (int j = 0; j < N; ++j) {

            if (j == N-1 || S.charAt(j) != S.charAt(j+1)) {

                // Here, [i, j] represents a group.

                if (j-i+1 >= 3)

                    ans.add(Arrays.asList(new Integer[]{i, j}));

                i = j + 1;

            }

        }

        return ans;

    }

https://leetcode.com/problems/positions-of-large-groups/discuss/128957/C++JavaPython-Straight-Forward

For every groups, find its start index i and end index j - 1
Group length is j - i, if it's no less than 3, add (i, j) to result.

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