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.

需考虑3类特殊情况：1、x1=x2，斜率无穷大；2、x1=x2,y1=y2，相同点也要计数，当计算最终结果时要加上该值，且相同点个数初始化为1，即其本身；3、y1=y2；

计算斜率时(double)(points[j].y-points[i].y)/(points[j].x-points[i].x)和(double)((points[j].y-points[i].y)/(points[j].x-points[i].x))结果不同，需注意。

 1 /**
 2  * Definition for a point.
 3  * struct Point {
 4  *     int x;
 5  *     int y;
 6  *     Point() : x(0), y(0) {}
 7  *     Point(int a, int b) : x(a), y(b) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     int maxPoints(vector<Point>& points) {
13         int n=points.size();
14         if(n<3) return n;
15         int res=0;
16         map<double,int> mp;
17         for(int i=0;i<n-1;i++)
18         {
19             int samepoint=1;
20             int maxpoint=0;
21             int samex=0;
22             
23             mp.clear();
24             map<double,int>::iterator it;
25             for(int j=i+1;j<n;j++)
26             {
27                 double slope=0.0;
28                 if(i==j)
29                     continue;
30                 if(points[i].x==points[j].x)
31                 {
32                     if(points[i].y==points[j].y)
33                         samepoint++;
34                     else
35                     {
36                         samex++;
37                         maxpoint=max(maxpoint,samex);
38                     }
39                     continue;
40                 }
41                 
42                 
43                 if(points[i].y==points[j].y)
44                     slope=0.0;
45                 else
46                     slope=(double)(points[j].y-points[i].y)/(points[j].x-points[i].x);
47                 it=mp.find(slope);
48                 if(it!=mp.end())
49                     it->second+=1;
50                 else
51                     mp.insert(pair<double,int>(slope,1));
52                 if(mp[slope]>maxpoint)   //if(it->second>maxpoint)
53                     maxpoint=mp[slope];  //maxpoint=it->second;  结果错误，为6452609，即it=mp.end(),空指针。
54             }
55             maxpoint+=samepoint;
56             res=max(res,maxpoint);
57         }
58         return res;
59     }
60 };