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
class Solution:
    def shiftingLetters(self, S: str, shifts: List[int]) -> str:      
        l = len(shifts)
        s = list(S)[:l]
        a = ord('a')
        
        # [1 + 3 + 5 + 7, 3 + 5 + 7, 5 + 7, 7]
        for i in range(1, len(shifts)):
            shifts[l - 1 - i] += shifts[l - i]
      
        print(shifts)
        
        for i in range(len(shifts)):
            # (ord(s[i]) - a + shifts[i]) % 26 이렇게 바꾼 숫자를 a에 더해줌
            s[i] = chr((ord(s[i]) - a + shifts[i]) % 26 + a)
        
        return ''.join(s)
class Solution:
    def shiftingLetters(self, S: str, shifts: List[int]) -> str:      
        l = len(shifts)
        s = list(S)[:l]
        a = ord('a')
        
        # [1 + 3 + 5 + 7, 3 + 5 + 7, 5 + 7, 7]
        for i in range(1, len(shifts)):
            shifts[l - 1 - i] += shifts[l - i]
      
        print(shifts)
        
        for i in range(len(shifts)):
            # (ord(s[i]) - a + shifts[i]) % 26 이렇게 바꾼 숫자를 a에 더해줌
            s[i] = chr((ord(s[i]) - a + shifts[i]) % 26 + a)
        
        return ''.join(s)