51nod1055 最长等差数列

#include<cstdio>
#include<cstring>
#include<cctype>
#include<algorithm>
using namespace std;
#define rep(i,s,t) for(int i=s;i<=t;i++)
#define dwn(i,s,t) for(int i=s;i>=t;i--)
int read(){
	int x=0;char c=getchar();
	while(!isdigit(c)) c=getchar();
	while(isdigit(c)) x=x*10+c-‘0‘,c=getchar();
	return x;
}
const int nmax=1e4+5;
short int dp[nmax][nmax];int a[nmax];
void maxs(int &a,int b){
	if(a<b) a=b;
}
int main(){
	int n=read();
	rep(i,1,n) a[i]=read();
	sort(a+1,a+n+1);
	int ans=0,j,k;
	rep(i,2,n-1){
		j=i-1,k=i+1;
		while(j&&k<=n){
			if(a[j]+a[k]>2*a[i]) --j;
			else if(a[j]+a[k]<2*a[i]) ++k;
			else{
				if(!dp[j][i]) dp[j][i]=dp[i][k]=3;
				else dp[j][i]=dp[i][k]=dp[j][i]+1;
				maxs(ans,dp[i][k]);--j;++k;
			}
		}
	}
	printf("%d\n",ans);
	return 0;
}