这个作业已经有点儿久远了，但是给我留下了很深的印象。这五周的作业中，就这个最困难了。自己真的花了两天时间在这次作业上面。最后解题思路还是在Coursera讨论上看到的。
brute-force解法就不多说了，但还是踩了一些坑。
重点在FastCollinear的实现，回过头来发现只有一百多行代码真的不敢置信。这次作业是对sort的应用。按每个点和其它点形成的斜率，然后排序。所以如果有四点在一个线上，非常容易寻找。但是最困的是找出重复的线段和子线段。在论坛找到的思路是遍历按斜率排序好的队列，将几个相同的放到Stack中，将保存到Stack中的同线点排序，于是就找出并保存了最大线段。
还有一个坑是，因为循环写法的原因，这个点的判断在下一次加入到队列中。所以最后一个点没有加入进去。而垂直的线斜率最大，正好排在最后。因此所有垂直的线丢没有找出来。
BruteCollinearPoints.java
···
import edu.princeton.cs.algs4.In;
import edu.princeton.cs.algs4.StdDraw;
import edu.princeton.cs.algs4.StdOut;

import java.util.Arrays;

public class BruteCollinearPoints {

private LineSegment[] segments;
private int endpoint;
private int nsegments;public BruteCollinearPoints(Point[] pointss) {if(pointss == null)throw new IllegalArgumentException();for(int i=0; i<pointss.length; i++) {if(pointss[i] == null)throw new IllegalArgumentException();}for(int i=0; i<pointss.length; i++) {for(int j=0; j<pointss.length; j++) {if(i == j)continue;if(pointss[i].slopeTo(pointss[j]) == Double.NEGATIVE_INFINITY)throw new IllegalArgumentException();}}Point[] points = new Point[pointss.length];for(int i=0;i<points.length;i++) {points[i] = pointss[i];}Arrays.sort(points);segments = new LineSegment[points.length];endpoint = points.length - 1;try {outer:for(int p=0; p<points.length; p++) {for(int q=endpoint; q>p; q--) {double pq = points[p].slopeTo(points[q]);for(int r=q-1; r>p; r--) {double qr = points[q].slopeTo(points[r]);if(qr == pq)for(int s=r-1; s>p; s--) {double rs = points[r].slopeTo(points[s]);if(qr == rs) {segments[nsegments++] = new LineSegment(points[p], points[q]);//continue outer;}}}}}} catch (NullPointerException e) {throw new IllegalArgumentException();}
}
public int numberOfSegments() {return nsegments;
}
public LineSegment[] segments() {LineSegment[] lineSegements = new LineSegment[nsegments];for(int i=0; i<nsegments; i++) {lineSegements[i] = segments[i];}return lineSegements;
}public static void main(String[] args) {// read the n points from a fileIn in = new In(args[0]);int n = in.readInt();Point[] points = new Point[n];for (int i = 0; i < n; i++) {int x = in.readInt();int y = in.readInt();points[i] = new Point(x, y);}// draw the pointsStdDraw.enableDoubleBuffering();StdDraw.setXscale(0, 32768);StdDraw.setYscale(0, 32768);for (Point p : points) {p.draw();}StdDraw.show();// print and draw the line segmentsBruteCollinearPoints collinear = new BruteCollinearPoints(points);for (LineSegment segment : collinear.segments()) {StdOut.println(segment);segment.draw();}StdDraw.show();
}

}

···
FastCollinearPoints.java

import edu.princeton.cs.algs4.In;
import edu.princeton.cs.algs4.StdDraw;
import edu.princeton.cs.algs4.StdOut;import java.util.Arrays;public class BruteCollinearPoints {private LineSegment[] segments;private int endpoint;private int nsegments;public BruteCollinearPoints(Point[] pointss) {if(pointss == null)throw new IllegalArgumentException();for(int i=0; i<pointss.length; i++) {if(pointss[i] == null)throw new IllegalArgumentException();}for(int i=0; i<pointss.length; i++) {for(int j=0; j<pointss.length; j++) {if(i == j)continue;if(pointss[i].slopeTo(pointss[j]) == Double.NEGATIVE_INFINITY)throw new IllegalArgumentException();}}Point[] points = new Point[pointss.length];for(int i=0;i<points.length;i++) {points[i] = pointss[i];}Arrays.sort(points);segments = new LineSegment[points.length];endpoint = points.length - 1;try {outer:for(int p=0; p<points.length; p++) {for(int q=endpoint; q>p; q--) {double pq = points[p].slopeTo(points[q]);for(int r=q-1; r>p; r--) {double qr = points[q].slopeTo(points[r]);if(qr == pq)for(int s=r-1; s>p; s--) {double rs = points[r].slopeTo(points[s]);if(qr == rs) {segments[nsegments++] = new LineSegment(points[p], points[q]);//continue outer;}}}}}} catch (NullPointerException e) {throw new IllegalArgumentException();}}public int numberOfSegments() {return nsegments;}public LineSegment[] segments() {LineSegment[] lineSegements = new LineSegment[nsegments];for(int i=0; i<nsegments; i++) {lineSegements[i] = segments[i];}return lineSegements;}public static void main(String[] args) {// read the n points from a fileIn in = new In(args[0]);int n = in.readInt();Point[] points = new Point[n];for (int i = 0; i < n; i++) {int x = in.readInt();int y = in.readInt();points[i] = new Point(x, y);}// draw the pointsStdDraw.enableDoubleBuffering();StdDraw.setXscale(0, 32768);StdDraw.setYscale(0, 32768);for (Point p : points) {p.draw();}StdDraw.show();// print and draw the line segmentsBruteCollinearPoints collinear = new BruteCollinearPoints(points);for (LineSegment segment : collinear.segments()) {StdOut.println(segment);segment.draw();}StdDraw.show();}
}