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:

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]]

遍历每个point，然后找和它等距离的其他point，按距离来保存，比如有一个点a，和它距离都是1的点有b，c，d，那么一共的组合就有6种，包括：[a, b, c], [a, c, b], [a, b, d], [a, d, b], [a, c, d], [a, d, c]。这么算是不考重复的情况下。还有可能b, c坐标完全相同，那么b和c会被当成两个点算两次。

public class Solution {
    public int numberOfBoomerangs(int[][] points) {       
        int result = 0;
        
        for(int i = 0; i < points.length; i++) {
            Map<Integer, Integer> map = new HashMap();
            for(int j = 0; j < points.length; j++) {
                if(i == j) continue;
                int dx = points[j][0] - points[i][0], dy = points[j][1] - points[i][1];
                int distance = dx * dx + dy * dy;

                map.put(distance, map.getOrDefault(distance, 0) + 1);
            }
            
            for(int k : map.keySet()) {
                int n = map.get(k);
                result += n * (n - 1);
            }
        }        
        return result;
    }
}

/**
 * @param {number[][]} points
 * @return {number}
 */
var numberOfBoomerangs = function(points) {
    const len = points.length
    if (len < 3) {
        return 0
    }
    let count = 0
    for (let i = 0; i < len; i++) {
        const point1 = points[i]
        // 用一个map存储其它点距离当前点point1有多少种距离可能
        const map = new Map()   
        for (let j = 0; j < len; j++) {
            if (i != j) {
                const point2 = points[j]
                const distance = getDistance(point1, point2)
                if (!map.has(distance)) {
                    map.set(distance, 1)
                } else {
                    map.set(distance, map.get(distance) + 1)
                }
            }
        }
        map.forEach(value => {
            if (value > 1) {
                const num = getArrangementNum(value)
                count += num
            }
        })
    }
    return count
};

// 计算两点之间距离，这里其实没必要开根号
function getDistance (p1, p2) {
    const distance = Math.sqrt(Math.pow(p1[0] - p2[0], 2) + Math.pow(p1[1] - p2[1], 2))
    return distance
}

// 计算排列数
function getArrangementNum (n) {
    return n * (n - 1)
}