<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 getLongestSeq(vector<int>& arr, int& diff, int i, int prev, vector<vector<int>>& dp){
        if(i >= arr.size()) return 0;

        if(dp[i][prev+1] != -1) return dp[i][prev+1];
        
        int pick = 0;
        int noPick = getLongestSeq(arr, diff, i+1 , prev, dp);
        if( prev == -1 || ((arr[i] - arr[prev]) == diff)){
            pick = 1 + getLongestSeq(arr, diff, i+1 ,i, dp);
        }
        return dp[i][prev+1] = max(pick, noPick);
    }

    int longestSubsequence(vector<int>& arr, int difference) {
        int n = arr.size();
        // vector<vector<int>> dp(n, vector<int>(n+1, -1));
        vector<int> dp(n, 1);
        map<int, int> mapping;
        int maxLen = INT_MIN;
        for(int i = 0; i < n; i++){
            if((mapping.find(arr[i] - difference) != mapping.end())){
                dp[i] = dp[mapping[arr[i] - difference]] + 1;
            }
            mapping[arr[i]] = i;
            maxLen = max(maxLen, dp[i]);
        }
        return maxLen;
    }
};
62000