Notes:

1. Do not forget to check xi == xj. It will cause INF.

2. DO not forget to initialize the iterator.

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
     
       &points) {13         int len = points.size(), duplicates = 1, result = 0;14         if (len < 3) return len;15         unordered_map
      
        mapping;16         for (int i = 0; i < points.size(); i++) {17             mapping.clear();18             mapping[INT_MIN] = 0;19             duplicates = 1;20             for (int j = 0; j < len; j++) {21                 if (i == j) continue;22                 if (points[i].x == points[j].x && points[i].y == points[j].y) {23                     duplicates++;24                     continue;25                 }26                 if (points[i].x == points[j].x) {27                     mapping[INT_MAX]++;28                     continue;29                 }30                 double k = double(points[i].y - points[j].y)/(points[i].x - points[j].x);31                 mapping[k]++;32             }33             for (unordered_map
       
        ::iterator it = mapping.begin(); it != mapping.end(); it++) {34                 result = max(result, it->second + duplicates);35             }36         }37         return result;38     }39 };