算法之奇偶交换排序

奇偶排序解开了冒泡排序每次迭代交换数据的相关性。它将排序分为两个过程，奇交换和偶交换。

• 对于奇交换来说，它总是比较奇数索引及其相邻的后续元素。

• 对于偶交换来说，它总是比较偶数索引及其相邻的后续元素。

• 奇交换和偶交换会成对出现，这样能保证比较和交换涉及数组中的每一个元素。

奇偶交换的串行实现

public static void oddEventSort(int[] arr){
    int exchFlag = 1, start = 0;
    while(exchFlag == 1 || start == 1){
        exchFlag = 0;
        for(int i = start; i<arr.length-1; i+=2){
            if(arr[i] > arr[i+1]){ 
                int temp = arr[i];
                arr[i] = arr[i+1];
                arr[i+1] = temp;
                exchFlag = 1;   
            }
        }
        if(0 == start){
            start = 1;
        }else{
          start = 0;  
        }
    }
}

奇偶交换的并行实现

static int[] arr = {1, 4, 5, 2, 3};
static ExecutorService pool = Executors.newCachedThreadPool();
static int exchFlag = 1;
static synchronized void setExchFlag(int v){
    exchFlag = v;
}
static synchronized int getExchFlag(){
    return exchFlag;
}
public static class oddEventSortTask implements Runnable {
    int i ;
    CountDownLatch latch;
    public oddEventSortTask(int i, CountDownLatch latch){
        this.i = i;
        this.latch =  latch;
    }
    @Override
    public void run(){
        if(arr[i] > arr[i+1]){
            int temp = arr[i];
            arr[i] = arr[i+1];
            arr[i+1] = temp;
            exchFlag = 1;
        }
        latch.countDown();
    }
}

public static void pOddEventSort(int[] arr) throws InterruptedException{
    int start = 0;
    while(1 == getExchFlag() || 1 == start){
        setExchFlag(0);
        //偶数的数组长度，当start为1时，只有len/2-1个线程
        CountDownLatch latch = new CountDownLatch(arr.length/2 - (arr.length%2 == 0?start : 0));
        for(int i =start; i<arr.length-1; i +=2){
            pool.submit(new oddEventSortTask(i, latch));
        }
        //等待所有线程结束
        latch.await();
        if(0 == start){
            start = 1;
        }else{
            start = 0;
        }
    }
}