leetcode 830

class Solution {
    
    
    public List<List<Integer>> largeGroupPositions(String s) {
    
    
        s += "#";
        List<List<Integer>> res = new ArrayList<>();

        int begin = 0;
        for (int i = 1; i < s.length(); i++) {
    
    
            if (s.charAt(i) != s.charAt(i - 1)){
    
    
                if (i - begin >= 3){
    
    
                    res.add(Arrays.asList(begin, i - 1));
                }
                begin = i;
            }
        }
        return res;
    }
}