# 二维最接近点对为什么要六等分

## Introduction

The two-dimensional closest pair problem is a classic problem in computational geometry. It involves computing the pair of points in a two-dimensional plane that are the closest to each other. The problem has a number of applications, including in robotics and computer vision. There are various algorithms that have been developed to solve this problem, but in this article, we will focus on one particular approach: the six-fold divide-and-conquer algorithm.

## The divide-and-conquer strategy

The divide-and-conquer strategy is a well-known algorithmic technique that involves breaking a problem down into smaller sub-problems and then solving these sub-problems independently. Once the solutions to the sub-problems have been obtained, they can be combined to solve the original problem. In the context of the closest pair problem, the divide-and-conquer strategy involves dividing the set of points in the plane into smaller subsets, computing the closest pairs within each subset, and then combining these solutions to obtain the overall closest pair.

## The six-fold divide-and-conquer algorithm

The six-fold divide-and-conquer algorithm for the closest pair problem is based on dividing the set of points into six equal subsets. To do this, the points are first sorted by their x-coordinates. The median x-coordinate is then computed, and the set of points is divided into three subsets: those that have x-coordinates less than the median, those that have x-coordinates equal to the median, and those that have x-coordinates greater than the median. The two subsets that have x-coordinates less than or equal to the median are then further subdivided into three subsets based on their y-coordinates. This gives a total of six subsets: one with x-coordinates less than the median, one with x-coordinates equal to the median, and four with x-coordinates greater than the median.

The closest pair within each subset is then computed recursively. Once all six subsets have been processed, the closest pair overall is found by comparing the closest pairs within each of the six subsets. The overall closest pair is then simply the pair that has the smallest distance among all the pairs considered.

## Performance of the six-fold divide-and-conquer algorithm

The six-fold divide-and-conquer algorithm has a time complexity of O(n log n), where n is the number of points in the plane. This is because at each level of the recursion, the number of subsets is six, and the size of each subset is reduced by a factor of two. The algorithm is therefore very efficient, particularly when the number of points is large.

## Conclusion

The six-fold divide-and-conquer algorithm is a powerful technique for solving the two-dimensional closest pair problem. By dividing the set of points into six equal subsets, the algorithm is able to compute the closest pair efficiently and accurately. The algorithm has a time complexity of O(n log n), making it very efficient when the number of points is large. If you need to solve the closest pair problem in a two-dimensional plane, the six-fold divide-and-conquer algorithm is definitely worth considering.

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