leetcode - 2381. Shifting Letters II

Description

You are given a string s of lowercase English letters and a 2D integer array shifts where shifts[i] = [starti, endi, directioni]. For every i, shift the characters in s from the index starti to the index endi (inclusive) forward if directioni = 1, or shift the characters backward if directioni = 0.

Shifting a character forward means replacing it with the next letter in the alphabet (wrapping around so that ‘z’ becomes ‘a’). Similarly, shifting a character backward means replacing it with the previous letter in the alphabet (wrapping around so that ‘a’ becomes ‘z’).

Return the final string after all such shifts to s are applied.

Example 1:

Input: s = "abc", shifts = [[0,1,0],[1,2,1],[0,2,1]]
Output: "ace"
Explanation: Firstly, shift the characters from index 0 to index 1 backward. Now s = "zac".
Secondly, shift the characters from index 1 to index 2 forward. Now s = "zbd".
Finally, shift the characters from index 0 to index 2 forward. Now s = "ace".

Example 2:

Input: s = "dztz", shifts = [[0,0,0],[1,1,1]]
Output: "catz"
Explanation: Firstly, shift the characters from index 0 to index 0 backward. Now s = "cztz".
Finally, shift the characters from index 1 to index 1 forward. Now s = "catz".

Constraints:

1 <= s.length, shifts.length <= 5 * 10^4
shifts[i].length == 3
0 <= starti <= endi < s.length
0 <= directioni <= 1
s consists of lowercase English letters.

Solution

Flatten shifts first, for start and end position, record the change of shifts. Then iterate the string from left to right, use cur_direction to keep track of how we should shift the character.

When we need to shift a 1 step forward or backward, because a does not start from 0, so remember to use chr - ord('a') first. To be more clear:
$$\text{newChar}=\text{‘a’ + (oldChar - ‘a’ + numberOfShifts) mod 26}$$

Time complexity: $o(n)$
Space complexity: $o(n)$

Code

class Solution:
    def shiftingLetters(self, s: str, shifts: List[List[int]]) -> str:
        direction_changes = {}
        for start, end, shift_dir in shifts:
            direction_changes[start] = direction_changes.get(start, 0) + 1 * (-1 if shift_dir == 0 else 1)
            direction_changes[end + 1] = direction_changes.get(end + 1, 0) + (-1) * (-1 if shift_dir == 0 else 1)
        res = []
        cur_direction = 0
        for i in range(len(s)):
            cur_direction += direction_changes.get(i, 0)
            new_char = chr((ord(s[i]) - ord('a') + cur_direction) % 26 + ord('a'))
            res.append(new_char)
        return ''.join(res)