[백준] 1157번 단어 공부

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
    	Scanner kb = new Scanner(System.in);
    	String sentence = kb.next().toUpperCase();
    	HashMap<Character, Integer> map = new HashMap<>();
    	List<Character> list = new ArrayList<>();
    	int max = 0;
    	
    	for(char c: sentence.toCharArray()) {
    		map.put(c, map.getOrDefault(c, 0)+1);
    		max = Math.max(max, map.get(c));
    	}
    	
    	for(Map.Entry<Character, Integer> entry: map.entrySet()) {
    		if(entry.getValue() == max) {
    			list.add(entry.getKey());
    		}
    	}
    	
    	if(list.size()>1) {
    		System.out.println("?");
    	}else {
    		System.out.println(list.get(0));
    	}
    }
}