#1141 : 二分·归并排序之逆序对（归并排序）

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#include<cstdio>
#include<algorithm>
//#include<bits/stdc++.h>
using namespace std;
template<class T>inline T read(T&x)
{
    char c;
    while((c=getchar())<=32)if(c==EOF)return 0;
    bool ok=false;
    if(c=='-')ok=true,c=getchar();
    for(x=0; c>32; c=getchar())
        x=x*10+c-'0';
    if(ok)x=-x;
    return 1;
}
template<class T> inline T read_(T&x,T&y)
{
    return read(x)&&read(y);
}
template<class T> inline T read__(T&x,T&y,T&z)
{
    return read(x)&&read(y)&&read(z);
}
template<class T> inline void write(T x)
{
    if(x<0)putchar('-'),x=-x;
    if(x<10)putchar(x+'0');
    else write(x/10),putchar(x%10+'0');
}
template<class T>inline void writeln(T x)
{
    write(x);
    putchar('\n');
}
//-------ZCC IO template------
const int maxn=100011;
const double inf=999999999;
#define lson (rt<<1),L,M
#define rson (rt<<1|1),M+1,R
#define M ((L+R)>>1)
#define For(i,t,n) for(int i=(t);i<(n);i++)
typedef long long  LL;
typedef double DB;
typedef pair<int,int> P;
#define bug printf("---\n");
#define mod 10007

LL ans=0;//int 是存不下的
void merge_sort(int *A,int x,int y,int *T)
{
    if(y-x>1)
    {
        int p=x,m=(x+y)/2;
        int i=x;
        int q=m;
        merge_sort(A,x,m,T);
        merge_sort(A,m,y,T);
        while(p<m||q<y)
        {
            if(q>=y||(p<m&&A[p]<=A[q]))T[i++]=A[p++];
            else {T[i++]=A[q++];ans+=m-p;}
        }
        for(int i=x;i<y;i++)
            A[i]=T[i];
    }
}
 int a[maxn];
 int b[maxn];
int main()
{
    //#ifndef ONLINE_JUDGE
   // freopen("in.txt","r",stdin);
    //#endif // ONLINE_JUDGE
    int n,m,i,j,k,t;
    while(read(n))
    {
        For(i,0,n)
        {
            read(a[i]);
        }
        ans=0;
        merge_sort(a,0,n,b);
        writeln(ans);
    }
    return 0;
}

#1141 : 二分·归并排序之逆序对（归并排序）

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原文地址：http://blog.csdn.net/u013167299/article/details/45014209