Is Minimum Additions to Make Valid String easy or hard?

The problem is rated Medium on LeetCode because it requires recognizing the repeating "abc" structure and handling sequence transitions correctly. The implementation itself is straightforward once you identify the greedy pattern validation strategy.

How to solve Minimum Additions to Make Valid String in O(n)?

Scan the string while maintaining the expected character in the repeating sequence a → b → c. When the current character does not match the expected one, simulate inserting the missing characters until the sequence aligns. Continue processing the string and add remaining characters at the end to complete the final "abc" group.

What is the best approach for Minimum Additions to Make Valid String?

The greedy sequence-validation approach is the most practical solution. You iterate through the string while tracking the expected character in the repeating "abc" sequence and count how many characters must be inserted when the order breaks. This runs in O(n) time with O(1) space and is typically the approach interviewers expect.

What data structure is used in Minimum Additions to Make Valid String?

The solution primarily relies on simple counters and character comparisons rather than complex data structures. Some implementations model the sequence using indices or modular arithmetic, while others track the expected character in the "abc" pattern.

What is the time complexity of Minimum Additions to Make Valid String?

The optimal solution runs in O(n) time where n is the length of the input string. Each character is processed exactly once while validating the expected "abc" order. The space complexity is O(1) because only a few counters or indices are stored.

Minimum Additions to Make Valid String Python or Java solution approach?

Both Python and Java implementations typically follow the greedy pattern-tracking approach. You iterate through the string, compare each character with the expected position in "abc", and increment a counter whenever characters must be inserted. The logic remains O(n) time and O(1) space across languages.

Is Minimum Additions to Make Valid String asked at Google, Amazon, or Meta?

String pattern validation and greedy sequence problems like this commonly appear in interviews at large tech companies including Google, Amazon, and Meta. While the exact problem may vary, the underlying idea of validating ordered character patterns in linear time is a common interview theme.

Description Hints 2 Solution Video 6:48

2645. Minimum Additions to Make Valid String

Medium50.8% acceptance3 companies

Given a string word to which you can insert letters "a", "b" or "c" anywhere and any number of times, return the minimum number of letters that must be inserted so that word becomes valid.

A string is called valid if it can be formed by concatenating the string "abc" several times.

Example 1:

Input: word = "b"
Output: 2
Explanation: Insert the letter "a" right before "b", and the letter "c" right next to "b" to obtain the valid string "abc".

Example 2:

Input: word = "aaa"
Output: 6
Explanation: Insert letters "b" and "c" next to each "a" to obtain the valid string "abcabcabc".

Example 3:

Input: word = "abc"
Output: 0
Explanation: word is already valid. No modifications are needed.

Constraints:

1 <= word.length <= 50
word consists of letters "a", "b" and "c" only.

Asked at 3 companies

Tokopedia

Google Microsoft

Topics

Hints open one at a time

Hint 1

Maintain a pointer on word and another pointer on string “abc”.

Hint 2

If the two characters that are being pointed to differ, Increment the answer and the pointer to the string “abc” by one.

6:48

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