447. Number of Boomerangs

Given n points in the plane that are all pairwise distinct, a "boomerang" is a tuple of points (i, j, k) such that the distance between iand j equals the distance between i and k (the order of the tuple matters).

Find the number of boomerangs. You may assume that n will be at most 500 and coordinates of points are all in the range [-10000, 10000] (inclusive).

Example:

Input:
[[0,0],[1,0],[2,0]]

Output:
2

Explanation:
The two boomerangs are [[1,0],[0,0],[2,0]] and [[1,0],[2,0],[0,0]]


给3个点，计算1个点到另外两个点都相等有多少种组合

hypot(x,y)   计算 x 与 y 平方和的平方根

C++(386ms):

class Solution {
public:
    int numberOfBoomerangs(vector<pair<int, int>>& points) {
        int res = 0;
        unordered_map<double, int> ctr(points.size());
        for (auto p : points) {
            for (auto q : points){
                if (p == q)
                    continue ;
                double t = hypot(p.first - q.first, p.second - q.second) ;
                ctr[t]++ ;
                res += 2 * (ctr[t]-1);
            }
            ctr.clear() ;
        }
        return res;
    }
};