Java几种常见算法的实现

以下是几种常见的算法在 Java 中的实现，涵盖了排序算法、查找算法和一些常用的算法范式。

1. 冒泡排序 (Bubble Sort)

public class BubbleSort {
    public static void bubbleSort(int[] arr) {
        int n = arr.length;
        for (int i = 0; i < n - 1; i++) {
            for (int j = 0; j < n - 1 - i; j++) {
                if (arr[j] > arr[j + 1]) {
                    // 交换
                    int temp = arr[j];
                    arr[j] = arr[j + 1];
                    arr[j + 1] = temp;
                }
            }
        }
    }

    public static void main(String[] args) {
        int[] arr = {64, 25, 12, 22, 11};
        bubbleSort(arr);
        System.out.println("排序后的数组:");
        for (int num : arr) {
            System.out.print(num + " ");
        }
    }
}

2. 选择排序 (Selection Sort)

public class SelectionSort {
    public static void selectionSort(int[] arr) {
        int n = arr.length;
        for (int i = 0; i < n - 1; i++) {
            int minIndex = i;
            for (int j = i + 1; j < n; j++) {
                if (arr[j] < arr[minIndex]) {
                    minIndex = j;
                }
            }
            // 交换
            int temp = arr[minIndex];
            arr[minIndex] = arr[i];
            arr[i] = temp;
        }
    }

    public static void main(String[] args) {
        int[] arr = {64, 25, 12, 22, 11};
        selectionSort(arr);
        System.out.println("排序后的数组:");
        for (int num : arr) {
            System.out.print(num + " ");
        }
    }
}

3. 插入排序 (Insertion Sort)

public class InsertionSort {
    public static void insertionSort(int[] arr) {
        int n = arr.length;
        for (int i = 1; i < n; i++) {
            int key = arr[i];
            int j = i - 1;

            // 将大于 key 的元素移到一位
            while (j >= 0 && arr[j] > key) {
                arr[j + 1] = arr[j];
                j = j - 1;
            }
            arr[j + 1] = key;
        }
    }

    public static void main(String[] args) {
        int[] arr = {64, 25, 12, 22, 11};
        insertionSort(arr);
        System.out.println("排序后的数组:");
        for (int num : arr) {
            System.out.print(num + " ");
        }
    }
}

4. 二分查找 (Binary Search)

public class BinarySearch {
    public static int binarySearch(int[] arr, int target) {
        int left = 0, right = arr.length - 1;

        while (left <= right) {
            int mid = left + (right - left) / 2;

            // 检查中间元素
            if (arr[mid] == target) {
                return mid;
            }

            // 如果目标元素大于中间元素
            if (arr[mid] < target) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }
        return -1; // 未找到
    }

    public static void main(String[] args) {
        int[] arr = {2, 3, 4, 10, 40};
        int target = 10;
        int result = binarySearch(arr, target);
        if (result == -1) {
            System.out.println("元素未找到");
        } else {
            System.out.println("元素找到在索引: " + result);
        }
    }
}

5. 快速排序 (Quick Sort)

public class QuickSort {
    public static void quickSort(int[] arr, int low, int high) {
        if (low < high) {
            // 找到基准元素
            int pi = partition(arr, low, high);

            // 分别对基准元素左边和右边进行快速排序
            quickSort(arr, low, pi - 1);
            quickSort(arr, pi + 1, high);
        }
    }

    public static int partition(int[] arr, int low, int high) {
        int pivot = arr[high];
        int i = (low - 1);
        for (int j = low; j < high; j++) {
            if (arr[j] <= pivot) {
                i++;
                // 交换 arr[i] 和 arr[j]
                int temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }

        // 交换 arr[i+1] 和 arr[high]
        int temp = arr[i + 1];
        arr[i + 1] = arr[high];
        arr[high] = temp;

        return i + 1;
    }

    public static void main(String[] args) {
        int[] arr = {10, 7, 8, 9, 1, 5};
        int n = arr.length;
        quickSort(arr, 0, n - 1);
        System.out.println("排序后的数组:");
        for (int num : arr) {
            System.out.print(num + " ");
        }
    }
}

6. 广度优先搜索 (BFS)

import java.util.*;

public class BFS {
    // 广度优先搜索实现
    public static void bfs(int[][] graph, int start) {
        boolean[] visited = new boolean[graph.length];
        Queue<Integer> queue = new LinkedList<>();

        visited[start] = true;
        queue.offer(start);

        while (!queue.isEmpty()) {
            int node = queue.poll();
            System.out.print(node + " ");

            for (int i = 0; i < graph[node].length; i++) {
                if (graph[node][i] == 1 && !visited[i]) {
                    visited[i] = true;
                    queue.offer(i);
                }
            }
        }
    }

    public static void main(String[] args) {
        // 图的邻接矩阵表示
        int[][] graph = {
            {0, 1, 0, 1},
            {1, 0, 1, 1},
            {0, 1, 0, 0},
            {1, 1, 0, 0}
        };

        System.out.println("广度优先搜索结果:");
        bfs(graph, 0);
    }
}

7. 深度优先搜索 (DFS)

import java.util.*;

public class DFS {
    // 深度优先搜索实现
    public static void dfs(int[][] graph, int start, boolean[] visited) {
        visited[start] = true;
        System.out.print(start + " ");

        for (int i = 0; i < graph[start].length; i++) {
            if (graph[start][i] == 1 && !visited[i]) {
                dfs(graph, i, visited);
            }
        }
    }

    public static void main(String[] args) {
        // 图的邻接矩阵表示
        int[][] graph = {
            {0, 1, 0, 1},
            {1, 0, 1, 1},
            {0, 1, 0, 0},
            {1, 1, 0, 0}
        };

        boolean[] visited = new boolean[graph.length];
        System.out.println("深度优先搜索结果:");
        dfs(graph, 0, visited);
    }
}