830. 较大分组的位置

class Solution {
public:
    vector<vector<int>> largeGroupPositions(string s) 
    {
        vector<vector<int>> ans;
        int n = s.size ();
        int num = 1;
        for(int i=0; i<n; i++)
        {
            if(i==n-1 || s[i]!=s[i+1])
            {
                if(num >= 3){
                    ans.push_back({i-num+1,i});
                }
                num = 1;
            }
            else{
                num++;
            }
        }
        return ans;
    }
};