2021.11.05 逆序对

2021.11.05 逆序对

[P1966 NOIP2013 提高组] 火柴排队 - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
#include<queue>
#include<map>
using namespace std;

//最大对最大？？？WF一道题？？？
#define int long long
const int N=1e6+10;
const int mod=1e8-3;
int n,a[N],b[N],ai[N],bi[N],mapi[N];
int tree[N];

inline int read(){
	int s=0,w=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){
		if(ch=='-')w=-1;
		ch=getchar();
	}
	while(ch<='9'&&ch>='0'){
		s=s*10+ch-'0';
		ch=getchar();
	}
	return s*w;
} 
inline int lowbit(int x){
	return x&-x;
}
inline void add(int x,int k){
	for(int i=x;i<=n;i+=lowbit(i))tree[i]+=k;
}
inline int query(int x){
	int fin=0;
	for(int i=x;i>0;i-=lowbit(i))fin+=tree[i];
	//cout<<fin;
	return fin;
}

signed main(){
	n=read();
	for(int i=1;i<=n;i++)ai[i]=a[i]=read();
	sort(ai+1,ai+n+1);
	int lena=unique(ai+1,ai+n+1)-ai-1;
	for(int i=1;i<=n;i++)a[i]=lower_bound(ai+1,ai+lena+1,a[i])-ai;
	for(int i=1;i<=n;i++)bi[i]=b[i]=read();
	sort(bi+1,bi+n+1);
	int lenb=unique(bi+1,bi+n+1)-bi-1;
	for(int i=1;i<=n;i++)b[i]=lower_bound(bi+1,bi+lenb+1,b[i])-bi;
	for(int i=1;i<=n;i++)mapi[a[i]]=i,a[i]=i;
	for(int i=1;i<=n;i++)b[i]=mapi[b[i]];
	//for(int i=1;i<=n;i++)cout<<b[i]<<" ";cout<<endl;
	int ans=0;
	for(int i=n;i>=1;i--){
		ans+=query(b[i]);ans%=mod;
		add(b[i],1);
		//for(int j=1;j<=10;j++)cout<<tree[j]<<"  ";
	}
	cout<<ans;
	return 0;
}

[P3531 POI2012]LIT-Letters - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)

这个神奇的写法之前似乎在那道题上用过，哈哈哈，忘了。十年OI一场空，不开long long见祖宗，泪目了。

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;

#define int long long
const int N=1e6+10;
int n,mapi[30][N],b[N],tree[N];

inline int read(){
	int s=0,w=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){
		if(ch=='-')w=-1;
		ch=getchar();
	}
	while(ch<='9'&&ch>='0'){
		s=s*10+ch-'0';
		ch=getchar();
	}
	return s*w;
} 
inline int lowbit(int x){
	return x&-x;
}
inline void add(int x,int k){
	for(int i=x;i<=n;i+=lowbit(i))tree[i]+=k;
}
inline int query(int x){
	int fin=0;
	for(int i=x;i>0;i-=lowbit(i))fin+=tree[i];
	return fin;
}

signed main(){
	n=read();
	for(int i=1;i<=n;i++){
		char ch;cin>>ch;
		int x=ch-'A'+1;
		++mapi[x][0];
		mapi[x][mapi[x][0]]=i;
	}
	for(int i=1;i<=26;i++)mapi[i][0]=0;
	for(int i=1;i<=n;i++){
		char ch;cin>>ch;
		int x=ch-'A'+1;
		++mapi[x][0];
		b[i]=mapi[x][mapi[x][0]];
	}
	//for(int i=1;i<=n;i++)cout<<b[i]<<" ";cout<<endl;
	int ans=0;
	for(int i=n;i>=1;i--){
		ans+=query(b[i]);
		add(b[i],1);
	}
	cout<<ans;
	return 0;
}