哈希表

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

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        PrintWriter pw = new PrintWriter(System.out);

        String str = br.readLine();
        Set<Character> set = new HashSet<>(1 << 8);
        for (char c : str.toCharArray()) {
            set.add(c);
        }
        pw.println(set.size());

        pw.flush();
        pw.close();
        br.close();
    }
}

也可以用数组代替哈希表，效率可能会高一点点

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

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        PrintWriter pw = new PrintWriter(System.out);

        String str = br.readLine();
        boolean[] record = new boolean[1 << 7];
        for (char c : str.toCharArray()) {
            record[c] = true;
        }
        int ans = 0;
        for (boolean r : record) {
            ans += r ? 1 : 0;
        }
        pw.println(ans);

        pw.flush();
        pw.close();
        br.close();
    }
}