TreeMap解决

import java.util.Scanner;
import java.util.TreeMap;

public class HJ8 {

    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        TreeMap<Integer,Integer> map = new TreeMap<>();
        int nums = sc.nextInt();

        for (int i = 0; i < nums; i++) {
            int key = sc.nextInt();
            int value = sc.nextInt();
            if(map.containsKey(key)){
                map.put(key,map.get(key) + value);
            }else{
                map.put(key,value);
            }
        }

        for(int key : map.keySet()){

            System.out.println(key + " " + map.get(key));
        }
    }
}