思路：

將點按照極角排序後，用滑動視窗法求解。

實現：

 1 class Solution
 2 {
 3 public:
 4     const double PI = acos(-1);
 5     int visiblePoints(vector<vector<int>>& points, int angle, vector<int>& location)
 6     {
 7         int cnt = 0;
 8         vector<double> v;
 9         for (auto& it: points)
 
10         {
11             if (it[0] == location[0] && it[1] == location[1])
12             {
13                 cnt++; continue;
14             }
15             double r = atan2(it[1] - location[1], it[0] - location[0]);
16             if (r < 0) r += 2 * PI;
17             r = r / PI * 180;
18
 
             v.push_back(r);
19         }
20         sort(v.begin(), v.end());
21         int n = v.size();
22         for (int i = 0; i < n; i++) v.push_back(v[i] + 360);
23         int l = 0, r = 0, res = 0;
24         while (r < 2 * n)
25         {
26             while (r < 2 * n && v[r] - v[l] <= angle) r++;
 
27             res = max(res, r - l);
28             while (r < 2 * n && l <= r && v[r] - v[l] > angle) l++;
29             r++;
30         }
31         return res + cnt;
32     }
33 }