POJ 刷题系列：2002. Squares

POJ 刷题系列：2002. Squares

传送门：2002. Squares

题意：

在二维坐标系下，给出若干点，求这些点最多能组成多少个正方形。

思路：
可以观察输入n只有1000，所以我们可以把这些点全部存入内存中（hash），因为已知正方形的两个点，可以求出另两个点的坐标。这样暴力枚举每个点，判断是否存在构成正方形的两个点。

代码如下：

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.StringTokenizer;

public class Main{

    String INPUT = "./data/judge/201801/P2002.txt";

    public static void main(String[] args) throws IOException {
        new Main().run();
    }

    static final int MOD = 100007;

    class P{
        int x;
        int y;
        P(int x, int y){
            this.x = x;
            this.y = y;
        }
    }

    List<P>[] ps;

    void hash(int x, int y) {
        int dist = (x * x + y * y) % MOD;
        if (ps[dist] == null) ps[dist] = new ArrayList<P>();
        ps[dist].add(new P(x, y));
    }

    boolean search(P a) {
        int dist = (a.x * a.x + a.y * a.y) % MOD;
        if (ps[dist] == null) return false;
        int size = ps[dist].size();
        for (int i = 0; i < size; ++i) {
            P b = ps[dist].get(i);
            if (b.x == a.x && b.y == a.y) return true;
        }
        return false;
    }


    void read() {
        while(true) {
            int n = ni();
            if (n == 0) break;
            ps = new ArrayList[MOD];
            int[][] points = new int[n][2];
            for (int i = 0; i < n; ++i) {
                points[i] = new int[] {ni(), ni()};
                hash(points[i][0], points[i][1]);
            }

            int ans = 0;
            for (int i = 0; i < n; ++i) {
                for (int j = i + 1; j < n; ++j) {
                    int x1 = points[i][0];
                    int y1 = points[i][1];
                    int x2 = points[j][0];
                    int y2 = points[j][1];

                    P pt3 = new P(x1 + y1 - y2, y1 + x2 - x1);
                    P pt4 = new P(x2 + y1 - y2, y2 + x2 - x1);
                    P pt5 = new P(x1 + y2 - y1, y1 + x1 - x2);
                    P pt6 = new P(x2 + y2 - y1, y2 + x1 - x2);

                    ans += search(pt3) && search(pt4) ? 1 : 0;
                    ans += search(pt5) && search(pt6) ? 1 : 0;
                }
            }

            out.println(ans / 4);
        }
    }

    FastScanner in;
    PrintWriter out;

    void run() throws IOException {
        boolean oj;
        try {
            oj = ! System.getProperty("user.dir").equals("F:\\oxygen_workspace\\Algorithm");
        } catch (Exception e) {
            oj = System.getProperty("ONLINE_JUDGE") != null;
        }

        InputStream is = oj ? System.in : new FileInputStream(new File(INPUT));
        in = new FastScanner(is);
        out = new PrintWriter(System.out);
        long s = System.currentTimeMillis();
        read();
        out.flush();
        if (!oj){
            System.out.println("[" + (System.currentTimeMillis() - s) + "ms]");
        }
    }

    public boolean more(){
        return in.hasNext();
    }

    public int ni(){
        return in.nextInt();
    }

    public long nl(){
        return in.nextLong();
    }

    public double nd(){
        return in.nextDouble();
    }

    public String ns(){
        return in.nextString();
    }

    public char nc(){
        return in.nextChar();
    }

    class FastScanner {
        BufferedReader br;
        StringTokenizer st;
        boolean hasNext;

        public FastScanner(InputStream is) throws IOException {
            br = new BufferedReader(new InputStreamReader(is));
            hasNext = true;
        }

        public String nextToken() {
            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (Exception e) {
                    hasNext = false;
                    return "##";
                }
            }
            return st.nextToken();
        }

        String next = null;
        public boolean hasNext(){
            next = nextToken();
            return hasNext;
        }

        public int nextInt() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Integer.parseInt(more);
        }

        public long nextLong() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Long.parseLong(more);
        }

        public double nextDouble() {
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return Double.parseDouble(more);
        }

        public String nextString(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more;
        }

        public char nextChar(){
            if (next == null){
                hasNext();
            }
            String more = next;
            next = null;
            return more.charAt(0);
        }
    }

    static class D{

        public static void pp(int[][] board, int row, int col) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < row; ++i) {
                for (int j = 0; j < col; ++j) {
                    sb.append(board[i][j] + (j + 1 == col ? "\n" : " "));
                }
            }
            System.out.println(sb.toString());
        }

        public static void pp(char[][] board, int row, int col) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < row; ++i) {
                for (int j = 0; j < col; ++j) {
                    sb.append(board[i][j] + (j + 1 == col ? "\n" : " "));
                }
            }
            System.out.println(sb.toString());
        }
    }

    static class ArrayUtils {

        public static void fill(int[][] f, int value) {
            for (int i = 0; i < f.length; ++i) {
                Arrays.fill(f[i], value);
            }
        }

        public static void fill(int[][][] f, int value) {
            for (int i = 0; i < f.length; ++i) {
                fill(f[i], value);
            }
        }

        public static void fill(int[][][][] f, int value) {
            for (int i = 0; i < f.length; ++i) {
                fill(f[i], value);
            }
        }
    }

    static class Num{
        public static <K> void inc(Map<K, Integer> mem, K k) {
            if (!mem.containsKey(k)) mem.put(k, 0);
            mem.put(k, mem.get(k) + 1);
        }
    }
}