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.

 * struct Point {

 *     int x;

 *     int y;

 *     Point() : x(0), y(0) {}

 *     Point(int a, int b) : x(a), y(b) {}

 * };

 */

class Solution {

public:

    bool isCollineation(Point& p1, Point& p2, Point& p3) //判断三点是否共线

    {

        int x1 = p1.x - p2.x ;

        int y1 = p1.y - p2.y ;

        int x2 = p1.x - p3.x ;

        int y2 = p1.y - p3.y ;

        if (x1 * y2 - x2 * y1 == 0)

            return true ;

        return false ;

    } ;

    int maxPoints(vector<Point> &points) 

    {

        int n=points.size();

        if(n<3) return n;

        

        int max=0;

        bool flagalldup=true;

        for(int i=0;i<n-2;i++)

        {

            int dup=0;

            for(int j=i+1;j<n-1;j++)

            {

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

                {

                    dup++;continue;

                }

                

                int line=2;

                for(int k=j+1;k<n;k++)

                {

                    flagalldup=false;

                    if(isCollineation(points[i],points[j],points[k]))

                        line=line+1;

                }

                line=line+dup;

                if(line>max) max=line;

            }

        }

        

        if(flagalldup) return n;

        else return max;

    }

};