Shifting Letters II — LeetCode 2381 Python Solution

The problem

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.

Example

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".

Python solution

class Solution:
    def shiftingLetters(self, s: str, shifts: List[List[int]]) -> str:
        n = len(s)
        d = [0] * (n + 1)
        for i, j, v in shifts:
            if v == 0:
                v = -1
            d[i] += v
            d[j + 1] -= v
        for i in range(1, n + 1):
            d[i] += d[i - 1]
        return ''.join(
            chr(ord('a') + (ord(s[i]) - ord('a') + d[i] + 26) % 26) for i in range(n)
        )

Complexity

Pattern: Prefix Sum

Precompute running totals once so any range query becomes a single subtraction. LeetCode 2381. Shifting Letters II is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Prefix Sum.

The prefix sum guide has the Python template for the pattern and the 157 LeetCode problems that use it.

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