POJ2299+逆序数

归并排序！！！！！！！！！！

/*
归并排序+求逆序数
*/
#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<stdlib.h>
using namespace std;
typedef __int64 int64;
const int maxn = 500005;
int64 a[ maxn ],Sort[ maxn ];
int64 res;

void init(){
    res = 0;
}

void merge( int L,int R ){
    int mid = (L+R)/2;
    int i = L;
    int j = mid+1;
    int pos = L;
    while( i<=mid&&j<=R ){
        if( a[i]<a[j] )
            Sort[ pos++ ] = a[i++];
        else{
            Sort[ pos++ ] = a[j++];
            res += (mid-i+1);
        }
    }
    while( i<=mid )
        Sort[ pos++ ] = a[i++];
    while( j<=R )
        Sort[ pos++ ] =  a[j++];
    for( int k=L;k<=R;k++ )
        a[k] = Sort[k];
}

void merge_sort( int L,int R ){
    int mid = (L+R)/2;
    if( L<R ){
        merge_sort( L,mid );
        merge_sort( mid+1,R );
        merge( L,R );
    }
    return ;
}

int main(){
    int n;
    while( scanf("%d",&n),n ){
        for( int i=1;i<=n;i++ )
            scanf("%I64d",&a[i]);
        init();
        merge_sort( 1,n );
        printf("%I64d
",res);
    }
    return 0;
}