848. Shifting Letters

#Problem

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.

https://leetcode.com/problems/shifting-letters/

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.

Example 2:

Input: s = "aaa", shifts = [1,2,3]
Output: "gfd"

Constraints:

• 1 <= s.length <= 10⁵
• s consists of lowercase English letters.
• shifts.length == s.length
• 0 <= shifts[i] <= 10⁹

#Test Cases

class Solution:
    def shiftingLetters(self, s: str, shifts: List[int]) -> str:

import pytest

from solution import Solution


@pytest.mark.parametrize('s, shifts, expected', [
    ("abc", [3,5,9], "rpl"),
    ("aaa", [1,2,3], "gfd"),
])
@pytest.mark.parametrize('sol', [Solution()])
def test_solution(sol, s, shifts, expected):
    assert sol.shiftingLetters(s, shifts.copy()) == expected

#Thoughts

第 i 个字符一共需要 shift Σshifts[:i+1] 次。先逆序遍历一遍 shifts，逐个向前累加即可。

#Code

class Solution:
    def shiftingLetters(self, s: str, shifts: list[int]) -> str:
        a = 97
        for i in range(len(s) - 2, -1, -1):
            shifts[i] += shifts[i + 1]

        return ''.join(chr(a + (ord(c) - a + k) % 26) for c, k in zip(s, shifts))