java分治思想之ForkJoin详解

public class Merge {
    public static void main(String[] args) {
        int[] arr = { 5, 2, 8, 4, 7, 1, 3, 6 };
        mergeSort(arr, 0, arr.length - 1);
        System.out.println(Arrays.toString(arr));
    }
    /**
     * 拆分
     * @param arr 数组
     * @param left 数组第一个元素
     * @param right 数组最后一个元素
     */
    public static void mergeSort(int[] arr,int left,int right){
        if (left < right) {
            int mid = (left + right) / 2;
            mergeSort(arr, left, mid);
            mergeSort(arr, mid + 1, right);
            merge(arr, left, mid, right);
        }
    }
    /**
     * 合并
     * @param arr 数组
     * @param left 数组第一个元素
     * @param mid 数组分割的元素
     * @param right 数组最后一个元素
     */
    public static void merge(int[] arr,int left,int mid,int right){
        //创建临时数组，用于存放合并后的数组
        int[] temp = new int[right - left + 1];
        //左面数组的起始指针
        int i = left;
        //右面数组的起始指针
        int j = mid + 1;
        //临时数组的下标
        int k = 0;
        //数组左面和数组右面都还有值就去遍历比较赋值
        while (i <= mid && j <= right) {
            //数组左面的值小于或者等于数组右面的值就把左面的值赋值到临时数组中
            //并且把左面的指针和临时数组的指针+1
            //否则相反
            if (arr[i] <= arr[j]) {
                temp[k] = arr[i];
                k++;
                i++;
            } else {
                temp[k] = arr[j];
                k++;
                j++;
            }
        }
        //把剩余数组塞进去
        while (i <= mid) {
            temp[k] = arr[i];
            k++;
            i++;
        }
        while (j <= right) {
            temp[k] = arr[j];
            k++;
            j++;
        }
        //讲临时数组中的元素拷贝会回原数组中
        for (int p = 0; p < temp.length; p++) {
            arr[left + p] = temp[p];
        }
    }
}

public class QuickSort {
    public static void main(String[] args) {
        int[] arr = { 5, 2, 8, 4, 7, 1, 3, 6 };
        quickSort(arr, 0, arr.length - 1);
        System.out.println(Arrays.toString(arr));
    }
    public static void quickSort(int[] arr,int left,int right){
        if(left<right){
            int pivotIndex  = partition(arr, left, right);
            quickSort(arr,left,pivotIndex-1);
            quickSort(arr,pivotIndex+1,right);
        }
    }
    public static int partition(int[] arr,int left,int right){
        //取第一个元素为基准元素
        int pivot = arr[left];
        int i = left+1;
        int j = right;
        while (i<=j){
            //当前指针位置数小于基准元素就继续移动指针直到遇到大的
            while (i<=j && arr[i] < pivot){
                i++;
            }
            //当前指针位置数大于基准元素就继续移动指针直到遇到小的
            while (i<=j && arr[j] > pivot){
                j--;
            }
            if(i<j){
                //交换元素位置
                swap(arr,i,j);
                i++;
                j--;
            }
        }
        //跳出循环说明i和j相遇,j的值一定是大于基准元素,要和基准元素进行交换
        swap(arr,left,j);
        //返回基准元素位置
        return j;
    }
    public static void swap(int[] array, int i, int j) {
        int temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    }
}

public class SumTask extends RecursiveTask<Integer> {
    private static final int THRESHOLD = 1000;
    private int[] array;
    private int start;
    private int end;
    public SumTask(int[] array, int start, int end) {
        this.array = array;
        this.start = start;
        this.end = end;
    }
    @Override
    protected Integer compute() {
        if (end - start <= THRESHOLD) {
            int sum = 0;
            for (int i = start; i < end; i++) {
                sum += array[i];
            }
            return sum;
        } else {
            int mid = (start + end) / 2;
            SumTask leftTask = new SumTask(array, start, mid);
            SumTask rightTask = new SumTask(array, mid, end);
            leftTask.fork();
            rightTask.fork();
            int leftResult = leftTask.join();
            int rightResult = rightTask.join();
            return leftResult + rightResult;
        }
    }
    public static void main(String[] args) {
        int[] array = new int[10000];
        for (int i = 0; i < array.length; i++) {
            array[i] = i + 1;
        }
        ForkJoinPool forkJoinPool = new ForkJoinPool();
        SumTask task = new SumTask(array, 0, array.length);
        int result = forkJoinPool.invoke(task);
        System.out.println(result);
    }
}