<p>Given an integer array <code>arr</code> and an integer <code>difference</code>, return the length of the longest subsequence in <code>arr</code> which is an arithmetic sequence such that the difference between adjacent elements in the subsequence equals <code>difference</code>.</p>

<p>A <strong>subsequence</strong> is a sequence that can be derived from <code>arr</code> by deleting some or no elements without changing the order of the remaining elements.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> arr = [1,2,3,4], difference = 1
<strong>Output:</strong> 4
<strong>Explanation: </strong>The longest arithmetic subsequence is [1,2,3,4].</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> arr = [1,3,5,7], difference = 1
<strong>Output:</strong> 1
<strong>Explanation: </strong>The longest arithmetic subsequence is any single element.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> arr = [1,5,7,8,5,3,4,2,1], difference = -2
<strong>Output:</strong> 4
<strong>Explanation: </strong>The longest arithmetic subsequence is [7,5,3,1].
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= arr.length &lt;= 10<sup>5</sup></code></li>
	<li><code>-10<sup>4</sup> &lt;= arr[i], difference &lt;= 10<sup>4</sup></code></li>
</ul>

class Solution {
public:
    int call(vector<int>& arr,int k,int i,int j,vector<vector<int>>& dp)
    {
        if(i>=arr.size())
        {
            return 0;
        }
        if(dp[i][j+1]!=-1)
        return dp[i][j+1];
        int l=call(arr,k,i+1,j,dp);
        if(j==-1 || arr[i]-arr[j]==k)
        l=max(l,1+call(arr,k,i+1,i,dp));
        return dp[i][j+1]=l;
    }
    int longestSubsequence(vector<int>& arr, int k) {
        // vector<vector<int>>dp(arr.size()+1,vector<int>(arr.size()+1,0));
        map<int,int>dp;
        // for(int i=arr.size()-1;i>=0;--i)
        // {
        //     for(int j=i-1;j>=-1;--j)
        //     {
        //         int l=dp[i+1][j+1];
        //         if(j==-1 || arr[i]-arr[j]==k)
        //         l=max(l,1+dp[i+1][i+1]);
        //         dp[i][j+1]=l;
        //     }
        // }
        int ans=0;
        for(int i=0;i<arr.size();++i)
        {
            int t=0;
            if(dp.find(arr[i]-k)!=dp.end())
            t=dp[arr[i]-k];
            dp[arr[i]]=1+t;
            ans=max(ans,dp[arr[i]]);
        }
        return ans;
    }
};
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