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);
}
}
}
