Most frequently asked interview question #2 with solution

We have a string S of lowercase letters, and an integer array shifts.
Call the shift of a letter, the next letter in the alphabet, (wrapping around so that 'z' becomes 'a').
For example, shift('a') = 'b', shift('t') = 'u', and shift('z') = 'a'.
Now for each shifts[i] = x, we want to shift the first i+1 letters of S, x times.
Return the final string after all such shifts to S are applied.

Solutions in C++:

string shiftingLetters(string S, vector<int>& shifts) {
            int sum = 0;
            int iter = S.length() - 1;
       
        for(auto it = shifts.rbegin(); it != shifts.rend(); ++it, --iter){
           
            sum = sum + *it;
            if(sum >= 26){
                sum = sum % 26;
            }
            *it = sum;

            S[iter]  = (char)(int('a') + (S[iter] + *it - (int)('a')) % 26);

        }
       
        return S;
    }