A linear approach to find largest substring between characters

 Problem is here: https://leetcode.com/problems/largest-substring-between-two-equal-characters/

1624. Largest Substring Between Two Equal Characters

Easy

Given a string s, return the length of the longest substring between two equal characters, excluding the two characters. If there is no such substring return -1.

A substring is a contiguous sequence of characters within a string.

 

Example 1:

Input: s = "aa"
Output: 0
Explanation: The optimal substring here is an empty substring between the two 'a's.

Example 2:

Input: s = "abca"
Output: 2
Explanation: The optimal substring here is "bc".

Example 3:

Input: s = "cbzxy"
Output: -1
Explanation: There are no characters that appear twice in s.

Example 4:

Input: s = "cabbac"
Output: 4
Explanation: The optimal substring here is "abba". Other non-optimal substrings include "bb" and "".

 

Constraints:

• 1 <= s.length <= 300
• s contains only lowercase English letters.

Solution can be accomplished in one pass: add to a hash map (or some other DS) the first occurrence of a character. If you see the character again, see if you have the max now. That's it. Code is below, cheers, ACC.

    public int MaxLengthBetweenEqualCharacters(string s)
    {
        if (String.IsNullOrEmpty(s)) return 0;

        Hashtable firstOccurrence = new Hashtable();

        int max = -1;
        for (int i = 0; i < s.Length; i++)
        {
            if (!firstOccurrence.ContainsKey(s[i])) firstOccurrence.Add(s[i], i);
            else max = Math.Max(max, i - (int)firstOccurrence[s[i]] - 1);
        }

        return max;
    }