Template to solve substring problems

import java.util.HashMap;
import java.util.Map;
 
public class SubstringTemplate {
 
    public int findSubstring(String s) {
        Map<Character, Integer> map = new HashMap<>();
        int counter = 0; // Check whether the substring is valid
        int begin = 0, end = 0; // Two pointers, one points to tail and one to head
        int d = 0; // The length of the substring
 
        // Initialize the hash map here
        for (char c : s.toCharArray()) {
            map.put(c, 0);
        }
 
        while (end < s.length()) {
            // Modify counter here
            if (map.containsKey(s.charAt(end)) && map.get(s.charAt(end))-- > 0) {
                counter--;
            }
            
 
            // Counter condition
            while (counter == 0) {
                // Update d here if finding minimum
                if (/* some condition */) {
                    d = end - begin;
                }
 
                // Increase begin to make it invalid/valid again
                if (map.containsKey(s.charAt(begin)) && map.get(s.charAt(begin))++ == 0) {
                    counter++;
                }
                begin++;
            }
            end++;
 
            // Update d here if finding maximum or minimum
        }
        return d;
    }
}