SGU 180

题型：求逆序数，（用树状数组+离散化）

心得：很悲剧，TLE AT #31 N 次，原因出在qsort()，（难怪发现不了），后来改sort()，就过了。看来qsort()的效率还有待分析！

代码

   
     

    
      
    
      #include 
    
      <
    
      stdio.h
    
      >
    
      
#include 
    
      <
    
      stdlib.h
    
      >
    
      
#include 
    
      <
    
      string
    
      .h
    
      >
    
      

    
      #define
    
       NL 65538
    
      

    
      #define
    
       LL __int64
    
      


    
      struct
    
       Num {
 
    
      int
    
       v, no;
}a[NL];

    
      int
    
       n, b[NL], mx;

    
      int
    
       t[NL];


    
      int
    
       cmp(
    
      const
    
       
    
      void
    
       
    
      *
    
      a, 
    
      const
    
       
    
      void
    
       
    
      *
    
      b)
{
 
    
      return
    
       ((
    
      struct
    
       Num 
    
      *
    
      )a)
    
      ->
    
      v 
    
      -
    
       ((
    
      struct
    
       Num 
    
      *
    
      )b)
    
      ->
    
      v;
}

inline 
    
      int
    
       lowbit(
    
      int
    
       k)
{
 
    
      return
    
       k
    
      &
    
      (
    
      -
    
      k);
}


    
      int
    
       sum(
    
      int
    
       k)
{
 
    
      int
    
       cnt 
    
      =
    
       
    
      0
    
      ;
 
    
      while
    
       (k 
    
      >
    
       
    
      0
    
      ) {
 cnt 
    
      +=
    
       t[k];
 k 
    
      -=
    
       lowbit(k);
 }
 
    
      return
    
       cnt;
}


    
      void
    
       update(
    
      int
    
       k, 
    
      int
    
       c)
{
 
    
      while
    
       (k 
    
      <=
    
       mx) {
 t[k] 
    
      +=
    
       c;
 k 
    
      +=
    
       lowbit(k);
 }
}


    
      int
    
       main()
{
 
    
      int
    
       i, p;
 LL cnt;
 
    
      //
    
       freopen("in.txt", "r", stdin);
 
    
      //
    
       freopen("out.txt", "w", stdout);
    
      

    
       scanf(
    
      "
    
      %d
    
      "
    
      , 
    
      &
    
      n);
 
    
      for
    
       (i
    
      =
    
      0
    
      ; i
    
      <
    
      n; i
    
      ++
    
      ) {
 scanf(
    
      "
    
      %d
    
      "
    
      , 
    
      &
    
      a[i].v);
 a[i].no 
    
      =
    
       i;
 }
 qsort(a, n, 
    
      sizeof
    
      (a[
    
      0
    
      ]), cmp);
 p
    
      =
    
      1
    
      ;
 b[a[
    
      0
    
      ].no] 
    
      =
    
       
    
      1
    
      ;
 
    
      for
    
       (i
    
      =
    
      1
    
      ; i
    
      <
    
      n; i
    
      ++
    
      ) {
 
    
      if
    
       (a[i].v 
    
      !=
    
       a[i
    
      -
    
      1
    
      ].v) p
    
      ++
    
      ;
 b[a[i].no] 
    
      =
    
       p;
 }
 mx 
    
      =
    
       p;
 memset(t, 
    
      0
    
      , 
    
      sizeof
    
      (t));
 cnt 
    
      =
    
       
    
      0
    
      ;
 
    
      for
    
       (i
    
      =
    
      n
    
      -
    
      1
    
      ; i
    
      >=
    
      0
    
      ; i
    
      --
    
      ) {
 cnt 
    
      +=
    
       sum(b[i]
    
      -
    
      1
    
      );
 update(b[i], 
    
      1
    
      );
 }
 printf(
    
      "
    
      %I64d\n
    
      "
    
      , cnt);
 
    
      return
    
       
    
      0
    
      ;
 
}