等差数列求和

typedef long long ll;
const int mod = 1e9+7;
class Solution {
public:
    int countHomogenous(string s) {
        int n=s.size();
        ll cnt=0;
        ll res=0;
        for(int i=0;i<n;i++)
        {
            if(i&&s[i]!=s[i-1])
            {
                res=(res+(1ll+cnt)*cnt/2)%mod;
                cnt=1;
            }
            else cnt++;
        }
        res=(res+(1ll+cnt)*cnt/2)%mod;
        return res;
    }
};