MaxPointsOnALine.java

/*
 Author:     Andy, nkuwjg@gmail.com
 Date:       Dec 03, 2014
 Problem:    Max Points On a Line
 Difficulty: Easy
 Notes:
 Given n points on a 2D plane, find the maximum number of points that lie on the same straight line.
 Solution: 1. hashmap. Time: O(n^2), Space: O(n);
*/

/**
 * 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) {
        int res = 0;
        int N = points.length;
        for (int i = 0; i < N; ++i) {
            HashMap<Double, Integer> m = new HashMap<Double, Integer>();
            int ss = 1, sp = 0;
            for (int j = i + 1; j < N; ++j) {
                double slope = Double.MIN_VALUE;
                if (points[i].x != points[j].x) {
                    slope = (double)(points[i].y - points[j].y) / (points[i].x - points[j].x);
                    if (slope == -0.0) slope = 0.0;
                } else if (points[i].y == points[j].y) {
                    sp += 1; continue;
                }
                int tmp = 2;
                if (m.containsKey(slope)) {
                    tmp = m.get(slope) + 1;
                }
                m.put(slope, tmp);
                ss = Math.max(ss, tmp);
            }
            res = Math.max(res, ss + sp);
        }
        return res; 
    }
}