LeetCode 848 字母移位 Shifting Letters

class Solution {
public:
    string shiftingLetters(string S, vector<int>& shifts) {
        int sum = 0;
        for(int i = shifts.size()- 1; i>=0; --i)
        {
            S[i] = (S[i] - 'a' + (sum + shifts[i])%26)%26 + 'a';
            sum = (sum + shifts[i])%26;
        }
        return S;
    }
};