void ihap(p,n)
  int n,p[];
  { int i,mm,t;
    void isift();
    mm=n/2;
    for (i=mm-1; i>=0; i--)
      isift(p,i,n-1);
    for (i=n-1; i>=1; i--)
      { t=p[0]; p[0]=p[i]; p[i]=t;
        isift(p,0,i-1);
      }
    return;
  }

  static void isift(p,i,n)
  int i,n,p[];
  { int j,t;
    t=p[i]; j=2*(i+1)-1;
    while (j<=n)
      { if ((j<n)&&(p[j]<p[j+1])) j=j+1;
        if (t<p[j])
          { p[i]=p[j]; i=j; j=2*(i+1)-1;}
        else j=n+1;
      }
    p[i]=t;
    return;
  }