KD树（K-Dimensional Tree）是一种用于多维空间的二叉树数据结构，旨在提供高效的数据检索。KD树在空间搜索和最近邻搜索等问题中特别有用，允许在高维空间中有效地搜索数据点。
重要性质
1.分割K维数据空间的数据结构
2.是一颗二叉树
3.切分维度上，左子树值小于右子树值

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>

// 定义二维点的结构体
struct Point2D {
    double x;
    double y;
    Point2D(double _x, double _y) : x(_x), y(_y) {}
};

// 定义KD树节点
struct KDTreeNode {
    Point2D point;
    KDTreeNode* left;
    KDTreeNode* right;

    KDTreeNode(Point2D _point) : point(_point), left(nullptr), right(nullptr) {}
};

class KDTree {
private:
    KDTreeNode* root;

    // 构建KD树的递归函数
    KDTreeNode* buildKDTree(std::vector<Point2D>& points, int depth) {
        if (points.empty()) {
            return nullptr;
        }

        // 选择轴线，交替选择x和y坐标
        int axis = depth % 2;

        // 按轴线排序点
        if (axis == 0) {
            std::sort(points.begin(), points.end(), [](const Point2D& a, const Point2D& b) {
                return a.x < b.x;
            });
        } else {
            std::sort(points.begin(), points.end(), [](const Point2D& a, const Point2D& b) {
                return a.y < b.y;
            });
        }

        // 选择中间点作为节点
        int median = points.size() / 2;
        KDTreeNode* node = new KDTreeNode(points[median]);

        // 递归构建左子树和右子树
        std::vector<Point2D> leftPoints(points.begin(), points.begin() + median);
        std::vector<Point2D> rightPoints(points.begin() + median + 1, points.end());

        node->left = buildKDTree(leftPoints, depth + 1);
        node->right = buildKDTree(rightPoints, depth + 1);

        return node;
    }

    // 在KD树中查找最近邻点的递归函数
    KDTreeNode* findNearestNeighbor(KDTreeNode* node, Point2D target, int depth, KDTreeNode* best, double& bestDistance) {
        if (node == nullptr) {
            return best;
        }

        // 计算当前节点到目标点的距离
        double currentDistance = distance(node->point, target);

        // 更新最近邻点和距离
        if (currentDistance < bestDistance) {
            best = node;
            bestDistance = currentDistance;
        }

        // 选择子树
        int axis = depth % 2;
        KDTreeNode* nearSubtree;
        KDTreeNode* farSubtree;
        if (axis == 0) {
            if (target.x < node->point.x) {
                nearSubtree = node->left;
                farSubtree = node->right;
            } else {
                nearSubtree = node->right;
                farSubtree = node->left;
            }
        } else {
            if (target.y < node->point.y) {
                nearSubtree = node->left;
                farSubtree = node->right;
            } else {
                nearSubtree = node->right;
                farSubtree = node->left;
            }
        }

        // 递归搜索更近的子树
        best = findNearestNeighbor(nearSubtree, target, depth + 1, best, bestDistance);

        // 如果可能，搜索更远的子树
        if (shouldSearchFarSubtree(node, target, bestDistance)) {
            best = findNearestNeighbor(farSubtree, target, depth + 1, best, bestDistance);
        }

        return best;
    }

    // 计算两点之间的欧几里得距离
    double distance(Point2D a, Point2D b) {
        double dx = a.x - b.x;
        double dy = a.y - b.y;
        return std::sqrt(dx * dx + dy * dy);
    }

    // 检查是否需要搜索更远的子树
    bool shouldSearchFarSubtree(KDTreeNode* node, Point2D target, double bestDistance) {
        int axis = node->point.x > target.x ? 0 : 1; // 如果轴线是x，则比较x坐标；如果轴线是y，则比较y坐标
        double nodeDistance = axis == 0 ? node->point.x - target.x : node->point.y - target.y;
        return nodeDistance * nodeDistance < bestDistance;
    }

public:
    KDTree(std::vector<Point2D>& points) {
        root = buildKDTree(points, 0);
    }

    // 查找最近邻点
    Point2D findNearestNeighbor(Point2D target) {
        double bestDistance = std::numeric_limits<double>::max();
        KDTreeNode* bestNode = findNearestNeighbor(root, target, 0, nullptr, bestDistance);
        return bestNode->point;
    }
};

int main() {
    // 创建一些二维点
    std::vector<Point2D> points = {
        {2.0, 3.0},
        {5.0, 4.0},
        {9.0, 6.0},
        {4.0, 7.0},
        {8.0, 1.0},
        {7.0, 2.0}
    };

    // 构建KD树
    KDTree kdTree(points);

    // 查找最近邻点
    Point2D target(9.0, 2.0);
    Point2D nearestNeighbor = kdTree.findNearestNeighbor(target);

    std::cout << "The nearest neighbor to (" << target.x << ", " << target.y << ") is (" << nearestNeighbor.x << ", " << nearestNeighbor.y << ")" << std::endl;

    return 0;
}