830. 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”.

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]]

Note: 1 <= S.length <= 1000

class Solution {
    public List<List<Integer>> largeGroupPositions(String S) {
        List<List<Integer>> list = new ArrayList<>();
        int len = S.length();
        int left = 0;
        for (int i = 0; i < len; ++ i){
            if (i == len - 1 ||S.charAt(i) != S.charAt(i+1)){
                if (i - left + 1 >= 3){
                    List<Integer> l = new ArrayList<>(Arrays.asList(new Integer[]{left, i}));
                    list.add(l);
                }
                left = i+1;
            }
        }
        return list;
    }
}