848. Shifting Letters

You are given a string s of lowercase English letters and an integer array shifts of the same length.

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.

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