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447. Number of Boomerangs 👎

  • Time: $O(n^2)$
  • Space: $O(n)$
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class Solution {
 public:
  int numberOfBoomerangs(vector<vector<int>>& points) {
    int ans = 0;

    for (const vector<int>& p : points) {
      unordered_map<int, int> distCount;
      for (const vector<int>& q : points) {
        const int dist = getDist(p, q);
        ++distCount[dist];
      }
      for (const auto& [_, freq] : distCount)
        ans += freq * (freq - 1);  // C(freq, 2)
    }

    return ans;
  }

 private:
  int getDist(const vector<int>& p, const vector<int>& q) {
    return pow(p[0] - q[0], 2) + pow(p[1] - q[1], 2);
  }
};

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class Solution {
  public int numberOfBoomerangs(int[][] points) {
    int ans = 0;

    for (int[] p : points) {
      Map<Integer, Integer> distCount = new HashMap<>();
      for (int[] q : points) {
        final int dist = (int) getDist(p, q);
        distCount.merge(dist, 1, Integer::sum);
      }
      for (final int freq : distCount.values())
        ans += freq * (freq - 1); // C(freq, 2)
    }

    return ans;
  }

  private double getDist(int[] p, int[] q) {
    return Math.pow(p[0] - q[0], 2) + Math.pow(p[1] - q[1], 2);
  }
}

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class Solution:
  def numberOfBoomerangs(self, points: List[List[int]]) -> int:
    ans = 0

    for x1, y1 in points:
      count = collections.Counter()
      for x2, y2 in points:
        ans += 2 * count[(x1 - x2)**2 + (y1 - y2)**2]
        count[(x1 - x2)**2 + (y1 - y2)**2] += 1

    return ans