447. Number of Boomerangs

• Difficulty: Easy

• Topics: Hash Table

• Similar Questions:

  • Line Reflection

Problem:

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]]

Solutions:

class Solution {
public:
    int numberOfBoomerangs(vector<vector<int>>& points) {
        int ret = 0;
        for (int i = 0; i < points.size(); ++i) {
            map<int, int> distances;
            for (int j = 0; j < points.size(); ++j) {
                if (i == j) continue;
                ++distances[computeDistance(points[i], points[j])];
            }
            for (auto it = distances.begin(); it != distances.end(); ++it) {
                ret += (it -> second) * (it ->second - 1);
            }
        }
        return ret;
    }

private:
    int computeDistance(vector<int>& p1, vector<int>& p2) {
        return (p1[0] - p2[0]) * (p1[0] - p2[0]) + (p1[1] - p2[1]) * (p1[1] - p2[1]);
    }

};