Shifting Letters – Java O(n)

class Solution {
    public String shiftingLetters(String S, int[] shifts) {
        int n = shifts.length;
        int prev = 0;
        char[] arr = S.toCharArray();
        for (int i = n - 1; i >= 0; i--) {
            prev += shifts[i] % 26;
            arr[i] = getNextChar(arr[i], prev);
        }
        return new String(arr);
    }

    public char getNextChar(char a, int diff) {
        return (char)((((a - 'a') + diff) % 26) + 'a');
    }
}