heapalgorithm.java

该演示程序最初来自Sun公司的Java网站。笔者添加了另外三个算法（插入

/**
 * A Heapsort demonstration algorithm
 * HeapAlgorithm.java, 19.04.97
 *
 * @author   Lars Marius Garshol
 * @version  1.00 - 19.04.97
 */

class HeapAlgorithm extends SortAlgorithm {
  void sort(int a[]) throws Exception {
    int i,tmp;    

    //----Step 1: Make a heap in linear time. Top of heap at top of array.
    //            Largest numbers at the top
    for (i=a.length/2; i>=0; i--) {
      perc_down(i,a,a.length-1);
    }      

    //----Step 2: Take out elements one by one, largest first, and put them at the bottom
    for (i=a.length-1; i>=0; i--) {
      delete_max(i,a); //Places max element at a[i]
    }
  }

  void delete_max(int ix, int a[]) throws Exception {
    int ret;

    ret=a[0]; //Return value
    a[0]=a[ix];
    perc_down(0,a,ix-1); //Percolate last value down
    a[ix]=ret;
  }
  
  //Have to go to i*2+1 and i*2+2 because we start at 0
  //lng is the last index we can touch, the end of the heap
  void perc_down(int ix, int a[], int lng) throws Exception {
    int i,tmp;

    tmp=a[ix];
    i=ix;
    while (i*2+1<=lng) {
      if (i*2+1==lng || a[i*2+1]>a[i*2+2]) {
        if (a[i*2+1]<tmp) break; //Position found
        a[i]=a[i*2+1];
        i=i*2+1;
      } else {
        if (a[i*2+2]<tmp) break; //Position found      
        a[i]=a[i*2+2];
        i=i*2+2;
      }

      pause(i,ix);
    }

    a[i]=tmp;
  }
}