#include <iostream>
#include <map>
using namespace std;
map<char,int> mp;
int main(){
  int n;
  string s;
  cin>>n>>s;
  int ans=0,cnt=0;
  for(int i=0;i<n;i++){
    for(int j=i;j<n&&!mp[s[j]];j++){
      mp[s[j]]++;
      cnt++;
    }
    mp.clear();
    ans=max(ans,cnt);
    cnt=0;
  }
  cout<<ans<<endl;
  return 0;
}

JAVA ac代码:

import java.util.*;
import java.util.StringTokenizer;
import java.io.*;

public class Main {

	public static void main(String[] args) {
		int n=in.nextInt();
		int left=0,ans=0;
		String s=in.next();
		Set<Character> set=new HashSet<>();
		for(int right=0;right<s.length();right++){
			char c=s.charAt(right);//看这个字符是什么
      //如果集合中有这个字符，则删除前面的字符，直至集合中没有这个字符
      while(set.contains(c)){
				set.remove(s.charAt(left));
				left++;
			}
      //加上现在的字符
			set.add(c);
      //两个指针计算距离
			ans=Math.max(ans,right-left+1);
		}
		out.println(ans);
		out.close();
	}
	
	

	static FastReader in=new FastReader();
	static PrintWriter out=new PrintWriter(System.out);
	static class FastReader{
		static BufferedReader br;
		static StringTokenizer st;
		FastReader(){
			br=new BufferedReader(new InputStreamReader(System.in));
		}
		String next() {
			String str="";
			while(st==null||!st.hasMoreElements()) {
				try {
					str=br.readLine();
				}catch(IOException e) {
					throw new RuntimeException(e);
				}
				st=new StringTokenizer(str);
			}
			return st.nextToken();
		}
		int nextInt() {
			return Integer.parseInt(next());
		}
		double nextDouble() {
			return Double.parseDouble(next());
		}
		long nextLong() {
			return Long.parseLong(next());
		}
	}
	
}