[leetcode: Python]447.Number of Boomerangs
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Title:
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:2Explanation:The two boomerangs are [[1,0],[0,0],[2,0]] and [[1,0],[2,0],[0,0]]
题目大意:
给定平面上的n个两两不同的点,一个“回飞镖”是指一组点(i, j, k)满足i到j的距离=i到k的距离(考虑顺序)
计算回飞镖的个数。你可以假设n最多是500,并且点坐标范围在 [-10000, 10000] 之内。
解题思路:
枚举点i(x1, y1),计算点i到各点j(x2, y2)的距离,并分类计数
利用排列组合知识,从每一类距离中挑选2个点的排列数 A(n, 2) = n * (n - 1)
将上述结果累加即为最终答案。
方法一:1135ms
class Solution(object): def numberOfBoomerangs(self, points): """ :type points: List[List[int]] :rtype: int """ ans = 0 for x1, y1 in points: dmap = collections.defaultdict(int) for x2, y2 in points: dmap[(x1 - x2) ** 2 + (y1 - y2) ** 2] += 1 for d in dmap: ans += dmap[d] * (dmap[d] - 1) return ans
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