main.go

// Source: https://leetcode.com/problems/total-characters-in-string-after-transformations-i
// Title: Total Characters in String After Transformations I
// Difficulty: Medium
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// You are given a string `s` and an integer `t`, representing the number of **transformations** to perform. In one **transformation**, every character in `s` is replaced according to the following rules:
//
// - If the character is `'z'`, replace it with the string `"ab"`.
// - Otherwise, replace it with the **next** character in the alphabet. For example, `'a'` is replaced with `'b'`, `'b'` is replaced with `'c'`, and so on.
//
// Return the **length** of the resulting string after **exactly** `t` transformations.
//
// Since the answer may be very large, return it **modulo** `10^9 + 7`.
//
// **Example 1:**
//
// ```
// Input: s = "abcyy", t = 2
// Output: 7
// Explanation:
// - **First Transformation (t = 1)**:
//   - `'a'` becomes `'b'`
//   - `'b'` becomes `'c'`
//   - `'c'` becomes `'d'`
//   - `'y'` becomes `'z'`
//   - `'y'` becomes `'z'`
//   - String after the first transformation: `"bcdzz"`
// - **Second Transformation (t = 2)**:
//   - `'b'` becomes `'c'`
//   - `'c'` becomes `'d'`
//   - `'d'` becomes `'e'`
//   - `'z'` becomes `"ab"`
//   - `'z'` becomes `"ab"`
//   - String after the second transformation: `"cdeabab"`
// - **Final Length of the string**: The string is `"cdeabab"`, which has 7 characters.
// ```
//
// **Example 2:**
//
// ```
// Input: s = "azbk", t = 1
// Output: 5
// Explanation:
// - **First Transformation (t = 1)**:
//   - `'a'` becomes `'b'`
//   - `'z'` becomes `"ab"`
//   - `'b'` becomes `'c'`
//   - `'k'` becomes `'l'`
//   - String after the first transformation: `"babcl"`
// - **Final Length of the string**: The string is `"babcl"`, which has 5 characters.
// ```
//
// **Constraints:**
//
// - `1 <= s.length <= 10^5`
// - `s` consists only of lowercase English letters.
// - `1 <= t <= 10^5`
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

const modulo = int(1e9 + 7)

func lengthAfterTransformations(s string, t int) int {
	// Count alphabets, reverse order
	counter := [26]int{}
	for _, ch := range s {
		counter['z'-ch]++
	}

	for i := range t {
		counter[(25+i)%26] += counter[i%26]
		counter[(25+i)%26] %= modulo
	}

	ans := 0
	for _, count := range counter {
		ans += count
		ans %= modulo
	}

	return ans
}