hdu 1239 Calling Extraterrestrial Intelligence Again

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 2189    Accepted Submission(s): 1175

         本题其实很简单就是一个一个尝试就行，但是应该注意几点

第一：本题数据比较大，快速的求出素数表

第二：因为p/q<=1，所以p得取值范围应该是1<=p<=sqrt(n);

第三：运用二分查找迅速的查找当前的数是否是素数

第四：因为本题是求p,q 使得p*q最大，所以应该从最大往下搜for(j=m;j>=1;j--)。直到j<z位置不用再往下搜直接退出

代码：

  
    

   
     
   
      1 
   
     #include
   
     <
   
     stdio.h
   
     >
   
     

   
      2 
   
     #include
   
     <
   
     math.h
   
     >
   
     

   
      3 
   
     #include
   
     <
   
     string
   
     .h
   
     >
   
     

   
      4 
   
      
   
     int
   
      prim[
   
     100005
   
     ],
   
     is
   
     [
   
     100005
   
     ];

   
      5 
   
      
   
     int
   
      num
   
     =
   
     0
   
     ;

   
      6 
   
     
   
     int
   
      find(
   
     int
   
      v)

   
      7 
   
     {

   
      8 
   
      
   
     int
   
      m;

   
      9 
   
      
   
     int
   
      l
   
     =
   
     1
   
     ,h
   
     =
   
     num;

   
     10 
   
      
   
     while
   
     (l
   
     <
   
     h)

   
     11 
   
      {

   
     12 
   
      m
   
     =
   
     (l
   
     +
   
     h)
   
     /
   
     2
   
     ;

   
     13 
   
      
   
     if
   
     (prim[m]
   
     ==
   
     v)

   
     14 
   
      
   
     return
   
      
   
     1
   
     ;

   
     15 
   
      
   
     if
   
     (prim[m]
   
     <
   
     v)

   
     16 
   
      l
   
     =
   
     m
   
     +
   
     1
   
     ;

   
     17 
   
      
   
     else
   
     

   
     18 
   
      h
   
     =
   
     m;

   
     19 
   
      }

   
     20 
   
      
   
     return
   
      
   
     0
   
     ;

   
     21 
   
     }

   
     22 
   
     
   
     int
   
      main()

   
     23 
   
     {

   
     24 
   
      
   
     int
   
      m,a,b,j,max,i,mark,boo[
   
     330
   
     ],k,x,zx,zi,z;
   
     double
   
      t;

   
     25 
   
      
   
     int
   
      s,e
   
     =
   
     (
   
     int
   
     )(sqrt(
   
     0.0
   
     +
   
     100000
   
     )
   
     +
   
     1
   
     );

   
     26 
   
      memset(
   
     is
   
     ,
   
     1
   
     ,
   
     sizeof
   
     (
   
     is
   
     ));

   
     27 
   
      prim[
   
     ++
   
     num]
   
     =
   
     2
   
     ;
   
     is
   
     [
   
     0
   
     ]
   
     =
   
     is
   
     [
   
     1
   
     ]
   
     =
   
     0
   
     ;

   
     28 
   
      
   
     for
   
     (i
   
     =
   
     4
   
     ;i
   
     <=
   
     100000
   
     ;i
   
     +=
   
     2
   
     )

   
     29 
   
      
   
     is
   
     [i]
   
     =
   
     0
   
     ;

   
     30 
   
      
   
     for
   
     (i
   
     =
   
     3
   
     ;i
   
     <
   
     e;i
   
     +=
   
     2
   
     )

   
     31 
   
      
   
     if
   
     (
   
     is
   
     [i])

   
     32 
   
      {

   
     33 
   
      prim[
   
     ++
   
     num]
   
     =
   
     i;

   
     34 
   
      
   
     for
   
     (s
   
     =
   
     i
   
     *
   
     2
   
     ,j
   
     =
   
     i
   
     *
   
     i;j
   
     <=
   
     100000
   
     ;j
   
     +=
   
     s)

   
     35 
   
      
   
     is
   
     [j]
   
     =
   
     0
   
     ;

   
     36 
   
      }

   
     37 
   
      
   
     for
   
     ( ;i
   
     <
   
     100000
   
     ;i
   
     +=
   
     2
   
     )

   
     38 
   
      
   
     if
   
     (
   
     is
   
     [i])

   
     39 
   
      prim[
   
     ++
   
     num]
   
     =
   
     i;

   
     40 
   
     

   
     41 
   
      
   
     while
   
     (scanf(
   
     "
   
     %d%d%d
   
     "
   
     ,
   
     &
   
     m,
   
     &
   
     a,
   
     &
   
     b)
   
     !=
   
     EOF)

   
     42 
   
      {

   
     43 
   
      mark
   
     =
   
     0
   
     ;z
   
     =
   
     0
   
     ;

   
     44 
   
      
   
     if
   
     (m
   
     ==
   
     0
   
      
   
     &&
   
      a
   
     ==
   
     0
   
      
   
     &&
   
      b
   
     ==
   
     0
   
     )

   
     45 
   
      
   
     break
   
     ;

   
     46 
   
      t
   
     =
   
     a
   
     *
   
     1.0
   
     /
   
     b;

   
     47 
   
      
   
     for
   
     (j
   
     =
   
     m;j
   
     >=
   
     1
   
     ;j
   
     --
   
     )

   
     48 
   
      {

   
     49 
   
      
   
     if
   
     (j
   
     <
   
     z)

   
     50 
   
      
   
     break
   
     ;

   
     51 
   
      max
   
     =
   
     (
   
     int
   
     )sqrt(
   
     0.0
   
     +
   
     j);

   
     52 
   
      
   
     for
   
     (i
   
     =
   
     max;i
   
     >=
   
     1
   
     ;i
   
     --
   
     )

   
     53 
   
      {

   
     54 
   
      x
   
     =
   
     j
   
     /
   
     i;

   
     55 
   
      
   
     if
   
     (x
   
     *
   
     t
   
     <=
   
     i 
   
     &&
   
      x
   
     >=
   
     i)

   
     56 
   
      {

   
     57 
   
      
   
     if
   
     (find(i)
   
     &&
   
     find(x))

   
     58 
   
      {

   
     59 
   
      
   
     if
   
     (x
   
     *
   
     i
   
     >
   
     z)

   
     60 
   
      {

   
     61 
   
      zx
   
     =
   
     x;

   
     62 
   
      zi
   
     =
   
     i;

   
     63 
   
      z
   
     =
   
     x
   
     *
   
     i;

   
     64 
   
      }

   
     65 
   
      

   
     66 
   
      }

   
     67 
   
      }

   
     68 
   
      }

   
     69 
   
      }

   
     70 
   
      printf(
   
     "
   
     %d %d\n
   
     "
   
     ,zi,zx);

   
     71 
   
      }

   
     72 
   
      
   
     return
   
      
   
     0
   
     ;

   
     73 
   
     }

   
     74 
   
     

   
     75 
   
     

   
     76 
   
     

   
     77 
   
     

   
     78 
   
     

   
     79