《算法》第二章部分程序 part 5

▶ 书中第二章部分程序，加上自己补充的代码，包括利用优先队列进行多路归并和堆排序

● 利用优先队列进行多路归并

package package01;

import edu.princeton.cs.algs4.IndexMinPQ;
import edu.princeton.cs.algs4.In;
import edu.princeton.cs.algs4.StdOut;

public class class01
{
    private class01() {}

    private static void merge(In[] streams)
    {
        int n = streams.length;
        IndexMinPQ<String> pq = new IndexMinPQ<String>(n);
        for (int i = 0; i < n; i++)                     // 初始化队列，每个流输入一个元素
        {
            if (!streams[i].isEmpty())
                pq.insert(i, streams[i].readString());
        }
        for (;!pq.isEmpty();)
        {
            StdOut.print(pq.minKey() + " ");            // 每当输出来自某个流的元素，就从该流插入一个元素
            int i = pq.delMin();
            if (!streams[i].isEmpty())
                pq.insert(i, streams[i].readString());
        }
        StdOut.println();
    }

    public static void main(String[] args)
    {
        int n = args.length;
        In[] streams = new In[n];                       // 初始化输入流，输入多个文件
        for (int i = 0; i < n; i++)
            streams[i] = new In(args[i]);
        merge(streams);
    }
}

● 堆排序

package package01;

import edu.princeton.cs.algs4.StdIn;
import edu.princeton.cs.algs4.StdOut;

public class class01
{
    private class01() {}

    public static void sort(Comparable[] pq)
    {
        int n = pq.length;
        for (int k = n / 2; k >= 1; k--)  // 建堆
            sink(pq, k, n);
        for (; n > 1;)                    // 每次将最大元素交换到最后，重新调整前面的部分
        {
            exch(pq, 1, n--);
            sink(pq, 1, n);
        }
    }

    private static void sink(Comparable[] pq, int k, int n)
    {
        for (int j = k << 1; j <= n; exch(pq, k, j), k = j, j = k << 1)
        {
            if (j < n && less(pq, j, j + 1))
                j++;
            if (!less(pq, k, j))
                break;
        }
    }

    private static boolean less(Comparable[] pq, int i, int j)
    {
        return pq[i - 1].compareTo(pq[j - 1]) < 0;
    }

    private static void exch(Object[] pq, int i, int j)
    {
        Object swap = pq[i - 1];
        pq[i - 1] = pq[j - 1];
        pq[j - 1] = swap;
    }

    private static void show(Comparable[] a)
    {
        for (int i = 0; i < a.length; i++)
            StdOut.println(a[i]);
    }

    public static void main(String[] args)
    {
        String[] a = StdIn.readAllStrings();
        class01.sort(a);
        show(a);
    }
}