human
Solve in Python:
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.
Example 1:
Input: S = "abc", shifts = [3,5,9]
Output: "rpl"
Explanation: 
We start with "abc".
After shifting the first 1 letters of S by 3, we have "dbc".
After shifting the first 2 letters of S by 5, we have "igc".
After shifting the first 3 letters of S by 9, we have "rpl", the answer.

Note:

1 <= S.length = shifts.length <= 20000
0 <= shifts[i] <= 10 ^ 9
 What is a the Python program you requested?
assistant
#Here is the Python program you requested.

class Solution:
    def shiftingLetters(self, S: str, shifts: List[int]) -> str:
        add=0
        arr=list(S)
        i=len(S)-1
        while i>=0:
            add+=shifts[i]
            arr[i] = chr((((ord(arr[i]) - 97) + add) % 26) + 97)
            i-=1
        return ''.join(arr)