C. Almost Arithmetical Progression (cf)

因为有重复的数，就让hi[]记录其值。go[]记录值对应的其中一个值的序列号。这样就把

具有相同值的数统一起来了。

昨天都是对的，今天发现自己提交的又被判成错误了，看来自己随便想的东东还是很容易有各种漏洞啊。

自己找了一下，也不知道有什么例子不对的。真不好意思

仅供小小的参考

#include<iostream>#include<cstdio>#include<string>#include<cstring>#include<algorithm>using namespace std;int go[1111111]={0};int  b[5000];int main(){int i,j,k;int n;cin>>n;int hi[5000];for (i=0;i<n;i++){cin>>hi[i];go[hi[i]]=i;}memset(b,false,sizeof(b));int dp[5000];int ans=0;for (i=0;i<n;i++){int cur=i;for (j=0;j<n;j++)dp[j]=0,b[j]=-1;for (j=i+1;j<n;j++){//cout<<b[go[hi[j]]]<<' '<<cur<<endl;if (hi[i]==hi[j]){cur=j;}else if (b[go[hi[j]]]<cur){//cout<<i<<j<<"FUck";b[go[hi[j]]]=j;dp[go[hi[j]]]+=2;//cout<<hi[j]<<' '<<dp[go[hi[j]]]<<endl;}}for (j=0;j<n;j++){if (cur>b[go[hi[j]]] && !(dp[go[hi[j]]]&1))dp[go[hi[j]]]++;//cout<<j<<' '<<b[go[hi[j]]]<<endl;ans=max(ans,dp[go[hi[j]]]);//cout<<"  "<<ans<<"  ";}}cout<<ans<<endl;return 0;}

C. Almost Arithmetical Progression

time limit per test

2 seconds

memory limit per test

256 megabytes

input

standard input

output

standard output

Gena loves sequences of numbers. Recently, he has discovered a new type of sequences which he called an almost arithmetical progression. A sequence is analmost arithmetical progression, if its elements can be represented as:

• a₁ = p, wherep is some integer;
• a_i = a_i - 1 + ( - 1)^i + 1·q(i > 1), whereq is some integer.

Right now Gena has a piece of paper with sequence b, consisting ofn integers. Help Gena, find there the longest subsequence of integers that is an almost arithmetical progression.

Sequence s₁,  s₂,  ...,  s_k is a subsequence of sequenceb₁,  b₂,  ...,  b_n, if there is such increasing sequence of indexesi₁, i₂, ..., i_k(1  ≤  i₁  <  i₂  < ...   <  i_k  ≤  n), thatb_ij  =  s_j. In other words, sequences can be obtained fromb by crossing out some elements.

Input

The first line contains integer n (1 ≤ n ≤ 4000). The next line contains n integersb₁, b₂, ..., b_n(1 ≤ b_i ≤ 10⁶).

Output

Print a single integer — the length of the required longest subsequence.

Sample test(s)

Input

23 5

Output

2

Input

410 20 10 30

Output

3

Note

In the first test the sequence actually is the suitable subsequence.

In the second test the following subsequence fits: 10, 20, 10.