POJ2299+逆序数

归并排序！！！！！！！！！！

 1 /*

 2 归并排序+求逆序数

 3 */

 4 #include<stdio.h>

 5 #include<string.h>

 6 #include<algorithm>

 7 #include<stdlib.h>

 8 using namespace std;

 9 typedef __int64 int64;

10 const int maxn = 500005;

11 int64 a[ maxn ],Sort[ maxn ];

12 int64 res;

13 

14 void init(){

15     res = 0;

16 }

17 

18 void merge( int L,int R ){

19     int mid = (L+R)/2;

20     int i = L;

21     int j = mid+1;

22     int pos = L;

23     while( i<=mid&&j<=R ){

24         if( a[i]<a[j] )

25             Sort[ pos++ ] = a[i++];

26         else{

27             Sort[ pos++ ] = a[j++];

28             res += (mid-i+1);

29         }

30     }

31     while( i<=mid )

32         Sort[ pos++ ] = a[i++];

33     while( j<=R )

34         Sort[ pos++ ] =  a[j++];

35     for( int k=L;k<=R;k++ )

36         a[k] = Sort[k];

37 }

38 

39 void merge_sort( int L,int R ){

40     int mid = (L+R)/2;

41     if( L<R ){

42         merge_sort( L,mid );

43         merge_sort( mid+1,R );

44         merge( L,R );

45     }

46     return ;

47 }

48 

49 int main(){

50     int n;

51     while( scanf("%d",&n),n ){

52         for( int i=1;i<=n;i++ )

53             scanf("%I64d",&a[i]);

54         init();

55         merge_sort( 1,n );

56         printf("%I64d\n",res);

57     }

58     return 0;

59 }

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