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 i and 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:

class Solution {
public:
    int numberOfBoomerangs(vector<pair<int, int> >& points) 
    {
        if(points.size()<3)
            return 0;
        int ret=0;
        for(int i=0;i<points.size();i++)
        {
            unordered_map<int,int>hashmap;
            for(int j=0;j<points.size();j++)
            {
                if(j==i)
                    continue;
                int dis=distance(points[i],points[j]);
                hashmap[dis]++;
            }
            for(auto it=hashmap.begin();it != hashmap.end();it++)
            {
                if(it->second>=2)
                    ret += it->second*(it->second-1);
            }
        }
        return ret;
    }
    int distance(pair<int, int> &a,pair<int, int> &b)
    {
        int x=(a.first-b.first)*(a.first-b.first);
        int y=(a.second-b.second)*(a.second-b.second);
        return x+y;
    }
};

 

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]]