Is Total Characters in String After Transformations I easy or hard?

The problem is rated Medium because the straightforward simulation quickly becomes infeasible as the string grows. The key difficulty is recognizing that character order does not matter and switching to a frequency-based dynamic programming model.

Total Characters in String After Transformations I Python/Java solution

Both Python and Java implementations typically use an integer array of size 26 to track character frequencies. Each iteration shifts counts for 'a' through 'y' and handles the special expansion rule for 'z'. After t iterations, summing the array returns the total number of characters.

How to solve Total Characters in String After Transformations I in O(n)?

Achieving near O(n) work requires avoiding explicit string simulation. Convert the input string into character frequencies, then compute how each letter contributes to the final length after t steps using a recurrence or precomputed growth values. Summing these contributions gives the final character count without constructing intermediate strings.

What is the best approach for Total Characters in String After Transformations I?

The most practical solution tracks how many times each letter appears using a 26-element frequency array. Each transformation shifts counts forward and handles the special rule where 'z' becomes 'ab'. This counting dynamic programming approach runs in O(n + 26t) time and O(26) space and avoids building the exponentially growing string.

Is Total Characters in String After Transformations I asked at Google/Amazon/Meta?

Problems involving repeated transformations, character counting, and state transitions appear frequently in interviews at companies like Google, Amazon, and Meta. While this exact problem may not always appear, the pattern of converting string evolution into frequency-based dynamic programming is common in interview rounds.

What data structure is used in Total Characters in String After Transformations I?

The core data structure is a fixed-size array or hash table storing counts for the 26 lowercase letters. This structure allows constant-time updates for each transformation step and eliminates the need to manipulate large strings.

What is the time complexity of Total Characters in String After Transformations I?

The typical optimized solution runs in O(n + 26t) time. Building the initial frequency array takes O(n), and each of the t transformations updates a fixed array of 26 letters. Space complexity remains O(26) because only the alphabet counts are stored.

Description Hints 1 Solution Video 23:30

3335. Total Characters in String After Transformations I

Medium45.7% acceptance4 companies

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 10⁹ + 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 <= 10⁵
s consists only of lowercase English letters.
1 <= t <= 10⁵

Asked at 4 companies

Mathworks Amazon

Google Meta

Topics

Hints open one at a time

Hint 1

Maintain the frequency of each character.

23:30

Still stuck?Read 3 approaches, or watch a walkthrough.

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1class Solution:
2    def lengthAfterTransformations(self, s: str, t: int) -> int:
3        

Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

"abcyy"
2

Input

"azbk"
1

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