gym 102082B dp

和51nod1055 一样；

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<string>
#include<cmath>
#include<map>
#include<set>
#include<vector>
#include<queue>
#include<bitset>
#include<ctime>
#include<time.h>
#include<deque>
#include<stack>
#include<functional>
#include<sstream>
//#include<cctype>
//#pragma GCC optimize(2)
using namespace std;
#define maxn 5005
#define inf 0x7fffffff
//#define INF 1e18
#define rdint(x) scanf("%d",&x)
#define rdllt(x) scanf("%lld",&x)
#define rdult(x) scanf("%lu",&x)
#define rdlf(x) scanf("%lf",&x)
#define rdstr(x) scanf("%s",x)
#define mclr(x,a) memset((x),a,sizeof(x))
typedef long long  ll;
typedef unsigned long long ull;
typedef unsigned int U;
#define ms(x) memset((x),0,sizeof(x))
const long long int mod = 1e9 + 7;
#define Mod 1000000000
#define sq(x) (x)*(x)
#define eps 1e-5
typedef pair<int, int> pii;
#define pi acos(-1.0)
//const int N = 1005;
#define REP(i,n) for(int i=0;i<(n);i++)
typedef pair<int, int> pii;

inline int rd() {
	int x = 0;
	char c = getchar();
	bool f = false;
	while (!isdigit(c)) {
		if (c == '-') f = true;
		c = getchar();
	}
	while (isdigit(c)) {
		x = (x << 1) + (x << 3) + (c ^ 48);
		c = getchar();
	}
	return f ? -x : x;
}

ll gcd(ll a, ll b) {
	return b == 0 ? a : gcd(b, a%b);
}
int sqr(int x) { return x * x; }

/*ll ans;
ll exgcd(ll a, ll b, ll &x, ll &y) {
	if (!b) {
		x = 1; y = 0; return a;
	}
	ans = exgcd(b, a%b, x, y);
	ll t = x; x = y; y = t - a / b * y;
	return ans;
}
*/

int n;
int a[maxn];
int dp[5005][5005];
int main()
{
	//	ios::sync_with_stdio(0);
	n = rd();
	for (int i = 1; i <= n; i++) {
		a[i] = rd();
	}
	sort(a + 1, a + 1 + n);
	for (int i = 1; i <= n; i++) {
		for (int j = 1; j <= n; j++)dp[i][j] = 2;
	}
	int ans = 2;
	for (int i = 1; i <= n - 1; i++) {
		int pre = i - 1;
		for (int j = i + 1; j <= n; j++) {
			while (pre > 0 && a[j] - a[i] > a[i] - a[pre])pre--;
			if (!pre)break;
			if (pre > 0 && a[j] - a[i] == a[i] - a[pre])
				dp[j][i] = max(dp[j][i], dp[i][pre] + 1);
			ans = max(ans, dp[j][i]);
		}
	}
	cout << ans << endl;
	return 0;
}