<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();
        vector<int> dp(n, 1);
        int ans = 1;

        unordered_map<int, int> hash;
        hash.emplace(arr[0], 0);

        for (int i = 1; i < n; ++i) {
            int v = arr[i] - difference;
            if (hash.count(v)) {
                int p = hash[v];
                dp[i] = max(dp[i], dp[p]+1);
                cout << "i : " << i << " " << dp[i] << endl;
            }
            if (hash.count(arr[i])) {
                int j = hash[arr[i]];
                if (dp[i] > dp[j]) {
                    hash.erase(arr[i]);
                    hash.emplace(arr[i], i);
                }
            } else {
                hash.emplace(arr[i], i);
            }
            
            ans = max(ans, dp[i]);
        }

        return ans;
    }
};
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