830. Positions of Large Groups(#1 ).java

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