class Solution {
public:
    vector<vector<int>> largeGroupPositions(string S) {
        vector<vector<int>>res;
        if(S.size()<2)
            return res;  
        vector<int>index={1,1};
        S+=" ";//标志位（使得最后一个符合要求的字符串输出）
        int begin=0,end=0;
        for(int i=1;i<S.size();i++){
            end=i;
            if(S[end]!=S[begin]){
                if(end-begin>=3){
                    index[0]=begin;
                    index[1]=end-1;
                    res.push_back(index);
                    begin=end;
                    
                }
                else begin=end;
            }
        }
        return res;
    }
};