Largest Substring Between Two Equal Characters

Largest Substring Between Two Equal Characters

1. What is this problem about?

The Largest Substring Between Two Equal Characters coding problem asks you to find the length of the longest substring that lies between two identical characters in a given string. The two equal characters themselves are not included in the length. If no such pair of equal characters exists, you return -1.

2. Why is this asked in interviews?

This "Easy" level question is often used by companies like Zoho to test a candidate's understanding of string traversal and Hash Tables. It measures how efficiently you can track the occurrences of characters. While it can be solved with a brute-force approach, interviewers are looking for a clean, single-pass O(N) solution using a map or a fixed-size array.

3. Algorithmic pattern used

This follows the Hash Table and String interview pattern. The strategy is to record the first index where each character appears. As you continue to iterate through the string, if you see a character that you've encountered before, you calculate the distance between the current index and the first index of that character. You maintain a running maximum of these distances.

4. Example explanation

String: "abca"

1. 'a' at index 0. Store: {a: 0}.
2. 'b' at index 1. Store: {a: 0, b: 1}.
3. 'c' at index 2. Store: {a: 0, b: 1, c: 2}.
4. 'a' at index 3. 'a' is already in the map.
   • Distance = current_index - first_index - 1
   • Distance = 3 - 0 - 1 = 2.
   • Max length = 2. The longest substring is "bc" with length 2.

5. Common mistakes candidates make

The most common mistake is a double-nested loop, which results in O(N²) time complexity. While acceptable for very short strings, it’s not optimal. Another error is including the characters themselves in the length calculation (e.g., returning 4 for "abca" instead of 2). Candidates also sometimes forget to initialize the maximum length to -1 to handle strings with all unique characters.

6. Interview preparation tip

For "String, Hash Table interview pattern" questions, always think about what state you need to preserve. Here, only the first occurrence matters. In other problems, you might need the last occurrence or a count. Clearly identifying what to store in your Hash Table is half the battle.