【BZOJ4391】[Usaco2015 dec]High Card Low Card（贪心）

【BZOJ4391】[Usaco2015 dec]High Card Low Card（贪心）

题面

BZOJ

题解

预处理前缀后缀的结果，中间找个地方合并就好了。

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<set>
using namespace std;
#define MAX 100100
inline int read()
{
	int x=0;bool t=false;char ch=getchar();
	while((ch<'0'||ch>'9')&&ch!='-')ch=getchar();
	if(ch=='-')t=true,ch=getchar();
	while(ch<='9'&&ch>='0')x=x*10+ch-48,ch=getchar();
	return t?-x:x;
}
int a[MAX],b[MAX],n,tot,ans;
bool vis[MAX];
int s1[MAX],s2[MAX];
set<int> S;
int main()
{
	n=read();
	for(int i=1;i<=n;++i)vis[a[i]=read()]=true;
	for(int i=1;i<=n+n;++i)if(!vis[i])b[++tot]=i;
	for(int i=1;i<=n;++i)S.insert(b[i]);
	for(int i=1;i<=n;++i)
	{
		set<int>::iterator it=S.upper_bound(a[i]);
		if(it!=S.end())s1[i]=s1[i-1]+1,S.erase(it);
		else s1[i]=s1[i-1];
	}
	S.clear();
	for(int i=1;i<=n;++i)S.insert(-b[i]);
	for(int i=n;i;--i)
	{
		set<int>::iterator it=S.upper_bound(-a[i]);
		if(it!=S.end())s2[i]=s2[i+1]+1,S.erase(it);
		else s2[i]=s2[i+1];
	}
	for(int i=0;i<=n;++i)ans=max(ans,s1[i]+s2[i+1]);
	printf("%d\n",ans);
	return 0;
}