Max Points on a Line

Given n points on a 2D plane, find the maximum number of points that lie on the same straight line.

 

/**

 * 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 Solution {

    public int maxPoints(Point[] points) {

        HashMap<Double, Integer> slopeMap = new HashMap<Double, Integer>();

        Integer result = 0;

        if(points == null || points.length == 0) return result;

        for(int i = 0; i < points.length; i ++){

            slopeMap.clear();

            Integer sameValue = 0;

            for(int j = i; j < points.length; j ++){

            //need to start from i, or should return result + 1. since we don't count point i itself

                if(points[i].x == points[j].x && points[i].y == points[j].y){

                    sameValue ++;

                }else{

                    Double slope = slopeForTwoPoint(points[i], points[j]);

                    if(slopeMap.containsKey(slope)){

                        slopeMap.put(slope, slopeMap.get(slope) + 1);

                    }else{

                        slopeMap.put(slope, 1);

                    }

                }

            }

            Iterator it = slopeMap.values().iterator();

            while(it.hasNext()){

                Integer val = (Integer)it.next();

                result = Math.max(val + sameValue, result);

            }

            

            //if all the points are same points

            result = Math.max(sameValue, result);

        }

        return result;

    }

    

    public Double slopeForTwoPoint(Point a, Point b){

    //hashmap can have null as key, so take use of this feature.

        if(a.x == b.x) return null;

    //a.y == b.y case

        else if(a.y == b.y) return (double)0;

        else return ((double)a.y - (double)b.y) / ((double)a.x - (double)b.x);

    }

}