归并排序之逆序对

推荐大佬博客：https://www.cnblogs.com/adelalove/p/8484644.html

本人水平有限，题解不到为处，请多多谅解

本蒟蒻谢谢大家观看

 

题目：NOIP2013火柴排队

 

 

code:

#include<bits/stdc++.h>
#pragma GCC optimize(3)
#define mod 99999997
using namespace std;
int n,c[100005],d[100005],ans;
inline int read(){
    int x=0,f=1;char ch=getchar();
    while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();}
    while(isdigit(ch)){x=(x<<1)+(x<<3)+(ch^48);ch=getchar();}
    return x*f;
}
struct node
{
    int n,id;
}a[100005],b[100005];
void msort(int l,int r)
{
    if(l>=r)
        return;
    int m=(l+r)/2;
    msort(l,m);
    msort(m+1,r);
    int i=l,j=m+1,k=l;
    while(i<=m && j<=r)
    {
        if(c[i]>c[j])
        {
            d[k++]=c[j++];
            ans+=m-i+1;//求逆序对 
            ans%=mod;//题目要求 
        }
        else
        {
            d[k++]=c[i++];
        }
    }
    while(i<=m) d[k++]=c[i++];
    while(j<=r) d[k++]=c[j++];
    for(i=l;i<=r;i++)
    {
        c[i]=d[i];
     //cout<<"c[i]= "<<c[i]<<endl;   
    }
}
bool mcmp(node x,node y)
{
    return x.n<y.n;
}
int main(){
    n=read();
    int i;
    for(i=1;i<=n;i++)
    {
        a[i].n=read();
        a[i].id=i;
    }
    for(i=1;i<=n;i++)
    {
        b[i].n=read();
        b[i].id=i;
        //cout<<"b[i].id 22 == "<<b[i].id<<endl;
    }
    sort(a+1,a+n+1,mcmp);
    sort(b+1,b+n+1,mcmp);
    for(i=1;i<=n;i++)
    {
    //    cout<<"b[i].id= "<<b[i].id<<endl;
        c[a[i].id]=b[i].id;
        //cout<<"a[i].id= "<<a[i].id<<endl;
        //cout<<"c[a[i].id]= "<<c[a[i].id]<<endl;//<<"   b[i].id= "<<b[i].id<<endl;
    }
    msort(1,n);
    //for(int i=1;i<=n;i++)
    //cout<<"c[i]= "<<c[i]<<endl;
    printf("%d\n",ans);
    return 0;
}