[Java] 백준 - 2166번: 다각형의 면적

7월 13일(목) - 기하학 (2166번)

2166번: 다각형의 면적

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문제점

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import java.util.StringTokenizer;

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

        int N = Integer.parseInt(br.readLine());
        for (int i = 1; i <= N; i++) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int x = Integer.parseInt(st.nextToken());
            int y = Integer.parseInt(st.nextToken());
            p[i] = new point(x,y);
        }

        long result = 0;
        for (int i = 2; i <= N - 1; i++) {
            result = result + ccw(p[1].x, p[1].y, p[i].x, p[i].y, p[i+1].x, p[i+1].y);
        }

        System.out.println(Math.round(result));
    }

    public static long ccw(long x1, long y1, long x2, long y2, long x3, long y3) {
        long temp = (x1 * y2) + (x2 * y3) + (x3 * y1);
        temp = temp - (y1 * x2) - (y2 * x3) - (y3 * x1);

        return (temp / 2);
    }

    public static class point {
        long x;
        long y;
        public point(long x, long y){
            this.x = x;
            this.y = y;
        }
    }

}

1. C언어와는 달리, 자바에서는 구조체가 없기때문에 class를 사용하려함

-> 최대한 class를 활용해보려했으나 NullPointerException이 발생하면서 실패

-> 나중에 다시 시도

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풀이

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import java.util.StringTokenizer;

public class Main {
    static int N;
    static Po[] p;
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        N = Integer.parseInt(br.readLine());
        p = new Po[N+1];
        for(int i=1; i<=N; i++) {
            StringTokenizer st = new StringTokenizer(br.readLine());
            int x = Integer.parseInt(st.nextToken());
            int y = Integer.parseInt(st.nextToken());
            p[i] = new Po(x,y);
        }
        long res = 0;
        for(int i=2; i<N; i++) {
            res += ccw(p[1], p[i], p[i+1]);
        }

        res = Math.abs(res);
        if(res % 2 == 0) {
            System.out.println(res/2+".0");
        }else
            System.out.println(res/2+".5");
    }

    public static long ccw(Po p1, Po p2, Po p3) {
        //CCW 공식 (x1y2+x2y3+x3y1)−(y1x2+y2x3+y3x1)
        return ((p1.x*p2.y) + (p2.x*p3.y) + (p3.x * p1.y)) - ((p1.y*p2.x) + (p2.y*p3.x) + (p3.y*p1.x));

    }

    public static class Po{
        long x;
        long y;
        public Po(long x, long y) {
            this.x=x;
            this.y=y;
        }
    }
}

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풀이출처 -> https://ukyonge.tistory.com/185