[JAVA] 시소짝꿍

문제 설명
어느 공원 놀이터에는 시소가 하나 설치되어 있습니다. 이 시소는 중심으로부터 2(m), 3(m), 4(m) 거리의 지점에 좌석이 하나씩 있습니다.
이 시소를 두 명이 마주 보고 탄다고 할 때, 시소가 평형인 상태에서 각각에 의해 시소에 걸리는 토크의 크기가 서로 상쇄되어 완전한 균형을 이룰 수 있다면 그 두 사람을 시소 짝꿍이라고 합니다. 즉, 탑승한 사람의 무게와 시소 축과 좌석 간의 거리의 곱이 양쪽 다 같다면 시소 짝꿍이라고 할 수 있습니다.
사람들의 몸무게 목록 weights이 주어질 때, 시소 짝꿍이 몇 쌍 존재하는지 구하여 return 하도록 solution 함수를 완성해주세요.

제한 사항
2 ≤ weights의 길이 ≤ 100,000
100 ≤ weights[i] ≤ 1,000
몸무게 단위는 N(뉴턴)으로 주어집니다.
몸무게는 모두 정수입니다.

풀이

import java.util.*;
class Solution {
    public long solution(int[] weights) {
        long answer = 0;
        Map<Integer, Integer> weight = new HashMap<>();
        for(int i=0; i<weights.length; i++){
            // 같을 때
           if(!weight.containsKey(weights[i])){
                weight.put(weights[i], 1);
           } else {
               int cnt = weight.get(weights[i]);
               answer += cnt;
               weight.put(weights[i], ++cnt);
           }
            
            // 배율 비교
            // 0.5배
            double mul = (double) weights[i]/2;
            if(mul%1==0){
                if(weight.containsKey((int) mul)){
                    int cnt = weight.get((int) mul);
               answer += cnt;
                }
            }
            // 2배
            mul = weights[i]*2;
            if(weight.containsKey((int) mul)){
               int cnt = weight.get((int) mul);
            answer += cnt;
            }
            
            // 3/2배
            mul = (double) weights[i]*3/2;
            if(mul%1==0){
                if(weight.containsKey((int) mul)){
                    int cnt = weight.get((int) mul);
               answer += cnt;
                }
            }
            
            // 2/3배
            mul = (double) weights[i]*2/3;
            if(mul%1==0){
                if(weight.containsKey((int) mul)){
                    int cnt = weight.get((int) mul);
               answer += cnt;
                }
            }
            
            // 4/3배
            mul = (double) weights[i]*4/3;
            if(mul%1==0){
                if(weight.containsKey((int) mul)){
                    int cnt = weight.get((int) mul);
               answer += cnt;
                }
            }
            
            // 3/4배
            mul = (double) weights[i]*3/4;
            if(mul%1==0){
                if(weight.containsKey((int) mul)){
                    int cnt = weight.get((int) mul);
               answer += cnt;
                }
            }
        }
        return answer;
    }
}