Total Characters in String After Transformations I

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Question

Approach

I approached this problem by tracking how many letters of each type exist instead of the full string. Each transformation updates the counts: every z adds two a, and all others move forward in the alphabet. After repeating this process t times, the sum of all counts gives the final length.

Implementation

class Solution:
    def lengthAfterTransformations(self, s: str, t: int) -> int:
        MOD = 10**9 + 7
        counts = [0] * 26

        for ch in s:
            counts[ord(ch) - ord('a')] += 1

        for _ in range(t):
            next_counts = [0] * 26
            for i in range(25):  # 'a' to 'y'
                next_counts[i + 1] = (next_counts[i + 1] + counts[i]) % MOD
            # For 'z'
            next_counts[0] = (next_counts[0] + counts[25] * 2) % MOD
            counts = next_counts

        return sum(counts) % MOD

But it didn't work. So, I used the editorial