CF286-C

 

C. Mr. Kitayuta, the Treasure Hunter

time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

 

The Shuseki Islands are an archipelago of 30001 small islands in the Yutampo Sea. The islands are evenly spaced along a line, numbered from 0 to 30000 from the west to the east. These islands are known to contain many treasures. There are n gems in the Shuseki Islands in total, and the i-th gem is located on island p_i.

Mr. Kitayuta has just arrived at island 0. With his great jumping ability, he will repeatedly perform jumps between islands to the east according to the following process:

• First, he will jump from island 0 to island d.
• After that, he will continue jumping according to the following rule. Let l be the length of the previous jump, that is, if his previous jump was from island prev to island cur, let l = cur - prev. He will perform a jump of length l - 1, l or l + 1 to the east. That is, he will jump to island (cur + l - 1), (cur + l) or (cur + l + 1) (if they exist). The length of a jump must be positive, that is, he cannot perform a jump of length 0 when l = 1. If there is no valid destination, he will stop jumping.

Mr. Kitayuta will collect the gems on the islands visited during the process. Find the maximum number of gems that he can collect.

 

Input

The first line of the input contains two space-separated integers n and d (1 ≤ n, d ≤ 30000), denoting the number of the gems in the Shuseki Islands and the length of the Mr. Kitayuta's first jump, respectively.

The next n lines describe the location of the gems. The i-th of them (1 ≤ i ≤ n) contains a integer p_i (d ≤ p₁ ≤ p₂ ≤ ... ≤ p_n ≤ 30000), denoting the number of the island that contains the i-th gem.

 

Output

Print the maximum number of gems that Mr. Kitayuta can collect.

 

Sample test(s)

input

4 10
10
21
27
27

output

3

input

8 8
9
19
28
36
45
55
66
78

output

6

input

13 7
8
8
9
16
17
17
18
21
23
24
24
26
30

output

4

 

Note

In the first sample, the optimal route is 0  →  10 (+1 gem)  →  19  →  27 (+2 gems)  → ...

In the second sample, the optimal route is 0  →  8  →  15  →  21 →  28 (+1 gem)  →  36 (+1 gem)  →  45 (+1 gem)  →  55 (+1 gem)  →  66 (+1 gem)  →  78 (+1 gem)  → ...

In the third sample, the optimal route is 0  →  7  →  13  →  18 (+1 gem)  →  24 (+2 gems)  →  30 (+1 gem)  → ...

 

状态转移方程 dp[pos][len]=max(dp[pos+len-1][len-1],dp[pos+len][len],dp[pos+len+1][len+1])+a[pos]

pos和len的范围都是1~30000故不可能开这么大的数组,故对于len>2000,由于状态数比较少,直接dfs就可以了

 

#include <iostream>

#include <string.h>

using namespace std;

int a[30010];

int n,d;

int Max=-1;

int dp[30010][2010];

int dfs(int l,int len)

{

    if(l>Max)

        return 0;

    int cnt;

    if(len>1)

    {



        cnt=max(max(dfs(l+len-1,len-1),dfs(l+len,len)),dfs(l+len+1,len+1))+a[l];

        cnt=max(cnt,a[l]);

    }

    return cnt;



}

int DP( int i , int l ) {

    if( i > Max ) return 0 ;

    if( dp[i][l] == -1 ) {

        int &tmp = dp[i][l] ; tmp = a[i];

        if( l > 1 ) tmp = max( tmp , a[i] + DP( i + l - 1 , l - 1 ) );

        tmp = max( tmp , a[i] + DP( i + l , l ) );

        tmp = max( tmp , a[i] + DP( i + l + 1 , l + 1 ) );

    }

    return dp[i][l];

}



int main()

{

    cin>>n>>d;

    memset( dp , -1 , sizeof dp ) ;

    for(int i=1;i<=n;i++)

    {

        int k;

        cin>>k;

        a[k]++;

        Max=max(Max,k);

    }

    if(d>2000)

        cout<<dfs(d,d)<<endl;

    else

        cout<<DP(d,d)<<endl;

    return 0;

}