Is Count the Number of Special Characters I easy or hard?

Count the Number of Special Characters I is categorized as an Easy problem. The challenge mainly tests basic string traversal and character tracking using sets or arrays. Once you recognize that you only need to check 26 possible letters, the optimal O(n) solution becomes straightforward.

Count the Number of Special Characters I Python/Java solution

In Python or Java, iterate through the string and store lowercase and uppercase characters separately using sets or boolean arrays. After the iteration, compare each lowercase letter with its uppercase equivalent and count matches. The implementation remains O(n) time with constant space.

How to solve Count the Number of Special Characters I in O(n)?

Traverse the string once and track lowercase and uppercase letters separately using sets or boolean arrays. After processing the string, check each character from 'a' to 'z' and count those whose uppercase counterpart also appeared. Constant-time membership checks keep the entire solution linear.

What is the best approach for Count the Number of Special Characters I?

The optimal approach uses a single pass through the string and tracks characters with either sets or fixed-size boolean arrays. Both solutions run in O(n) time and O(1) space because the alphabet size is limited to 26 letters. The boolean array method is slightly more efficient due to direct indexing without hash operations.

Is Count the Number of Special Characters I asked at Google/Amazon/Meta?

Problems involving character tracking and case comparison appear frequently in interviews at companies like Amazon, Google, and Meta. While this exact problem may vary, the underlying pattern—using hash sets or arrays to track character presence—is a common interview topic.

What data structure is used in Count the Number of Special Characters I?

The typical data structures are hash sets or fixed-size boolean arrays. Sets provide easy membership checks, while arrays allow direct indexing using character offsets. Both support constant-time lookups, which is necessary to keep the algorithm O(n).

What is the time complexity of Count the Number of Special Characters I?

The time complexity is O(n), where n is the length of the string. You scan the string once to record which characters appear, and then check at most 26 letter pairs. Because the alphabet size is constant, the second step adds only constant overhead.

Description Hints 1 Solution Video 14:54

3120. Count the Number of Special Characters I

Easy77.2% acceptance3 companies

You are given a string word. A letter is called special if it appears both in lowercase and uppercase in word.

Return the number of special letters in word.

Example 1:

Input: word = "aaAbcBC"

Output: 3

Explanation:

The special characters in word are 'a', 'b', and 'c'.

Example 2:

Input: word = "abc"

Output: 0

Explanation:

No character in word appears in uppercase.

Example 3:

Input: word = "abBCab"

Output: 1

Explanation:

The only special character in word is 'b'.

Constraints:

1 <= word.length <= 50
word consists of only lowercase and uppercase English letters.

Asked at 3 companies

Gep Amazon

Google

Topics

Hash Table String

Hints open one at a time

Hint 1

The constraints are small. For all 52 characters, check if they are present in word.

14:54

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

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3120. Count the Number of Special Characters I

Asked at 3 companies

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