828

class Solution {
public:
    int uniqueLetterString(string s) {
        int index[26][2], ans, mod = int(1e9)+7;
        int n = s.length();
        ans = 0;
        memset(index, -1, sizeof(index));
        for(int i=0; i<n; i++){
            int c = s[i]-'A';
            ans = (ans + ((i-index[c][1])*(index[c][1]-index[c][0]))%mod)%mod;
            index[c][0] = index[c][1];
            index[c][1] = i;
        }

        for(int i=0; i<26; i++)
            ans = (ans + ((n-index[i][1])*(index[i][1]-index[i][0]))%mod)%mod;
        return ans;
    }
};