BZOJ 3357--[Usaco2004]等差数列(STL&DP)

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<queue>
#include<vector>
#include<map>
#define pa pair<LL,LL>
#define LL long long
using namespace std;
inline int read(){
    int x=0,f=1;char ch=getchar();
    while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}
    while(ch>=‘0‘&&ch<=‘9‘){x=x*10+ch-‘0‘;ch=getchar();}
    return x*f;
}
inline void Out(int a){
    if(a>9) Out(a/10);
    putchar(a%10+‘0‘);
}
const LL inf=1e9+10;
const LL mod=1e9+7;
const int N=2050;
int n,ans,a[N];
map<int,int> f[N]; 
int main(){
	n=read();
	if(n==1){
		printf("1\n");return 0;
	}
	for(int i=1;i<=n;++i)
		a[i]=read();
	for(int i=1;i<=n;++i){
		for(int j=1;j<i;++j){
			f[i][a[j]]=max(f[i][a[j]],2);
			f[i][a[j]]=max(f[i][a[j]],f[j][a[j]+a[j]-a[i]]+1);
			ans=max(ans,f[i][a[j]]);
		}
	}
	printf("%d\n",ans);
	return 0;
}