Fix issue in MaxPointsOnALine

Record both slope and intercept to identify a line.

Author: mengli

MaxPointsOnALine.java

@@ -1,47 +1,92 @@
 
 
 /**
- * Given n points on a 2D plane, find the maximum number of points that lie on the same straight line.
+ * Given n points on a 2D plane, find the maximum number of points
+ * that lie on the same straight line.
  */
 
 import java.util.HashMap;
 import java.util.Map;
 
+/**
+ * Definition for a point.
+ * class Point {
+ *     int x;
+ *     int y;
+ *     Point() { x = 0; y = 0; }
+ *     Point(int a, int b) { x = a; y = b; }
+ * }
+ */
 public class MaxPointsOnALine {
+
+	private class Line {
+		private final double mSlope;
+		private final double mIntercept;
+
+		public Line(double slope, double intercept) {
+			mSlope = slope;
+			mIntercept = intercept;
+		}
+
+		@Override
+		public boolean equals(Object object) {
+			if (this == object) {
+				return true;
+			} else if (object == null) {
+				return false;
+			} else if (object instanceof Line) {
+				Line line = (Line) object;
+				if (Math.abs(mSlope - line.mSlope) < 0.0001
+						&& Math.abs(mIntercept - line.mIntercept) < 0.0001) {
+					return true;
+				}
+			}
+			return false;
+		}
+
+		@Override
+		public int hashCode() {
+			final int prime = 31;
+			return prime * Double.toString(mSlope).hashCode()
+					+ Double.toString(mIntercept).hashCode();
+		}
+	}
 	public int maxPoints(Point[] points) {
-		Map<Double, Integer> map = new HashMap<Double, Integer>();
+		Map<Line, Integer> map = new HashMap<Line, Integer>();
 		int ret = 0;
 		int size = points.length;
 		for (int i = 0; i < size; i++) {
-			int invalidK = 0;
-			int add = 1;
+			int dup = 0;
 			for (int j = i + 1; j < size; j++) {
+				Line line = null;
 				if (points[j].x == points[i].x) {
 					if (points[j].y == points[i].y) {
-						add++;
-					} else {
-						invalidK++;
+						dup++;
+						continue;
 					}
-					continue;
+					line = new Line(Double.MAX_VALUE, Double.MAX_VALUE);
+				} else {
+					double slope = points[j].y == points[i].y ? 0.0
+							: (1.0 * (points[j].y - points[i].y))
+									/ (points[j].x - points[i].x);
+					double intercept = points[i].y - slope * points[i].x;
+					line = new Line(slope, intercept);
 				}
-				double k = points[j].y == points[i].y ? 0.0
-						: (1.0 * (points[j].y - points[i].y))
-								/ (points[j].x - points[i].x);
-				if (map.containsKey(k)) {
-					int count = map.get(k);
-					map.put(k, count + 1);
+				if (map.containsKey(line)) {
+					map.put(line, map.get(line) + 1);
 				} else {
-					map.put(k, 1);
+					map.put(line, 2);
 				}
 			}
-			for (Integer it : map.values()) {
-				if (it + add > ret) {
-					ret = it.intValue() + add;
-				}
+			for (Integer count : map.values()) {
+				ret = Math.max(ret, count.intValue() + dup);
 			}
-			ret = Math.max(invalidK + add, ret);
 			map.clear();
 		}
-		return ret;
+		return ret > 0 ? ret : points.length;
+	}
+
+	public static void main(String[] args) {
+		System.out.println(new MaxPointsOnALine().maxPoints(new Point[] {new Point(0,0),new Point(0,0)}));
 	}
 }