<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 longestSubsequence(vector<int>& arr, int difference) {
        int n=arr.size();
        int maxLength=1;
        /*vector<unordered_map<int, int>> dp(n);
        for(int j=1; j<n; j++) {
            for(int i=0; i<j; i++) {
                if(dp[i].count(difference)) {
                    dp[j][difference] = dp[i][difference] + 1;
                }
                else{
                    dp[j][difference] = 0;
                }
                maxLength = max(maxLength, dp[j][difference]);
            }
        }
    return maxLength;

        int index=1;
        int count =0;
        for(int i=0; i<n; i++) {
            index=1;
            count=1;
            for(int j=i+1; j<n; j++) {
                if((arr[j]-arr[i]) == index*difference) {
                    count++;
                    index++;
            }
        maxLength=max(maxLength, count);
        }
        }
    return maxLength;
    }*/

    unordered_map<int, int> dp(n);

    /*for(int i=0; i<n; i++) {
        dp[arr[i]]=1;
    }*/

    for(int i=0; i<n; i++) {
        if(dp.count(arr[i]-difference)) {
            dp[arr[i]] = dp[arr[i]-difference] + 1;
        }
        else{
            dp[arr[i]]=1;
        }
        maxLength=max(maxLength, dp[arr[i]]);
    }
    return maxLength;
    }
};
60900