Is Minimum Operations to Transform String easy or hard?

This problem is typically classified as Medium. The implementation is simple, but recognizing that consecutive mismatches can be grouped into one greedy operation is the key insight.

Minimum Operations to Transform String Python/Java solution

The implementation is short in most languages. Loop through the string, check if the current characters differ, and increment a counter when a new mismatch segment starts. The same logic works in Python, Java, C++, Go, and TypeScript with O(n) complexity.

How to solve Minimum Operations to Transform String in O(n)?

Iterate through the strings from left to right. Whenever s[i] differs from t[i] and the previous position matched (or i == 0), increment the operation count because a new mismatch block begins. Continue scanning; all characters in that block are handled by the same operation.

What is the best approach for Minimum Operations to Transform String?

The best approach is a greedy single-pass scan. Traverse both strings and count how many contiguous segments of mismatched characters appear. Each segment can be fixed with a single operation, giving an O(n) time and O(1) space solution.

Is Minimum Operations to Transform String asked at Google/Amazon/Meta?

Greedy string transformation problems appear frequently in interviews at large tech companies such as Amazon, Google, and Meta. Variants involving grouping mismatches or minimizing operations over contiguous segments are common screening questions.

What data structure is used in Minimum Operations to Transform String?

The solution mainly relies on simple string traversal with indices. No advanced data structure is required; the algorithm uses a greedy strategy while comparing characters directly.

What is the time complexity of Minimum Operations to Transform String?

The optimal solution runs in O(n) time because it scans the strings once while checking for the start of mismatch segments. Space complexity is O(1) since only a few counters and indices are used.

Description Hints 3 Solution Video 7:50

3675. Minimum Operations to Transform String

Medium61.9% acceptance

You are given a string s consisting only of lowercase English letters.

You can perform the following operation any number of times (including zero):

Choose any character c in the string and replace every occurrence of c with the next lowercase letter in the English alphabet.

Return the minimum number of operations required to transform s into a string consisting of only 'a' characters.

Note: Consider the alphabet as circular, thus 'a' comes after 'z'.

Example 1:

Input: s = "yz"

Output: 2

Explanation:

Change 'y' to 'z' to get "zz".
Change 'z' to 'a' to get "aa".
Thus, the answer is 2.

Example 2:

Input: s = "a"

Output: 0

Explanation:

The string "a" only consists of 'a' characters. Thus, the answer is 0.

Constraints:

1 <= s.length <= 5 * 10⁵
s consists only of lowercase English letters.

Topics

Greedy String

Hints open one at a time

Hint 1

Each operation shifts every occurrence of a chosen character forward by one in the alphabet (with wrap-around).

Hint 2

For any character c, the number of moves required to turn it into 'a' is (26 - (ord(c) - ord('a'))) % 26.

Hint 3

You can plan operations so characters that need more shifts are advanced first and cause merges that don't increase the total number of moves; therefore the minimum number of moves equals the maximum, over characters appearing in s, of (26 - (ord(c) - ord('a'))) % 26.

7:50

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

Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

"yz"

Input

"a"

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