LeetCode #447 - Number of Boomerangs

题目描述：

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:

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]]

求矩阵中回旋镖的数目，回旋镖的定义为三个点的组合，满足一个点到另外两个点的距离相等。首先需要计算每个点到其他点的距离，如果有n个点到这个点的距离相等，那么就有n*(n-1)个回旋镖。

class Solution {
public:
    int numberOfBoomerangs(vector<pair<int, int>>& points) {
        int count=0;
        for(int i=0;i<points.size();i++)
        {
            unordered_map<int,int> hash;
            for(int j=0;j<points.size();j++)
            {
                if(i==j) continue;
                int distance=pow(points[i].first-points[j].first,2)+pow(points[i].second-points[j].second,2);
                if(hash.count(distance)==0) hash[distance]=1;
                else hash[distance]++;
            }
            for(unordered_map<int,int>::iterator it=hash.begin();it!=hash.end();it++)
            {
                int n=(*it).second;
                if(n>=2) count+=n*(n-1);
            }
        }
        return count;
    }
};