hdu--1394--加强版的逆序数

 1 // 归并排序
 2 
 3 #include <iostream>
 4 #include <cstring>
 5 using namespace std;
 6 
 7 const int size = 5020;
 8 int sum;
 9 int arr[size];
10 int temp[size];
11 int cnt[size];
12 
13 void mergeSort( int x , int y )
14 {
15     int mid , p , q , i;
16     if( y - x >1 )
17     {
18         mid = x + (y-x) / 2;
19         p = x;
20         q = mid;
21         i = x;
22         mergeSort( x , mid );
23         mergeSort( mid , y );
24         while( p<mid || q<y )
25         {
26             if( q>=y || ( p<mid && arr[p] <= arr[q]) )
27             {
28                 temp[i++] = arr[p++];
29             }
30             else
31             {
32                 temp[i++] = arr[q++];
33                 sum+=(mid-p);
34             }
35         }
36         for( i = x ; i<y ; i++ )
37         {
38             arr[i] = temp[i];
39         }
40     }
41 }
42 
43 int main()
44 {
45     int n , mmin;
46     int p , q , num;
47     while( ~scanf("%d",&n) )
48     {
49         sum = 0;
50         for( int i = 0 ; i<n ; i++ )
51         {
52             scanf( "%d",&arr[i] );
53             cnt[i] = arr[i];
54         }
55         mergeSort( 0 , n );
56          mmin = sum; 
57         for( int i = 0 ; i<n-1 ; i++ )
58         {
59             p = q = 0;
60             num = cnt[0];
61             for( int j = 1 ; j < n ; j++ )
62             {
63                 cnt[j-1]=cnt[j];
64                 if( num<cnt[j] )
65                 {
66                     p++;
67                 }
68                 else if( num>cnt[j] ) 
69                 {
70                     q++;
71                 }
72             }
73             sum+=(p-q);
74             if(sum<mmin) 
75             {
76                 mmin = sum;
77             }
78             cnt[n-1]=num;
79         }
80         printf( "%d\n",mmin );
81     }
82     return 0;
83 }