2166 다각형의 면적 - 백준

https://www.acmicpc.net/problem/2166

2166번: 다각형의 면적

 

풀이

ccw 로 삼각형 구해서 다 더하면 된다.

코드

package test2166;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.util.StringTokenizer;
 
public class Main {
 
    static int N;
    static long x[];
    static long y[];
 
    public static void main(String[] args) throws NumberFormatException, IOException {
        // TODO Auto-generated method stub
        MyScanner sc = new MyScanner(System.in);
 
        N = sc.nextInt();
        x = new long[N + 1];
        y = new long[N + 1];
        for (int i = 0; i < N; i++) {
            x[i] = sc.nextLong();
            y[i] = sc.nextLong();
        }
        x[N] = x[0];
        y[N] = y[0];
 
        System.out.println(String.format("%.1f",ccw(x, y)));
 
        sc.close();
    }
 
    static double ccw(long[] x, long[] y) {
        long resultX = 0;
        long resultY = 0;
        for (int i = 0; i < N; i++) {
            resultX += x[i] * y[i + 1];
            resultY += x[i + 1] * y[i];
        }
 
        return (double)(Math.abs(resultX - resultY)) / 2;
    }
 
    static class MyScanner {
        final BufferedReader reader;
        final BufferedWriter writer;
        static StringTokenizer tokenizer = null;
 
        MyScanner(InputStream in) {
            reader = new BufferedReader(new InputStreamReader(in));
            writer = new BufferedWriter(new OutputStreamWriter(System.out));
        }
 
        String nextToken() throws IOException {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                tokenizer = new StringTokenizer(reader.readLine());
            }
 
            return tokenizer.nextToken();
        }
 
        int nextInt() throws NumberFormatException, IOException {
            return Integer.parseInt(nextToken());
        }
 
        long nextLong() throws NumberFormatException, IOException {
            return Long.parseLong(nextToken());
        }
 
        void write(String sb) throws IOException {
            writer.write(sb);
        }
 
        void close() throws IOException {
            reader.close();
            writer.close();
        }
    }
}