参考代码：

class Solution {
public:
    int longestSubsequence(vector<int>& arr, int difference) {
        int n=arr.size();
        unordered_map<int,int> hash;//nums[i]绑定dp[i]
        hash[arr[0]]=1;
        int ret=1;
        for(int i=1;i<n;i++)
        {
            int a=arr[i];
            int b=a-difference;
            hash[a]=hash[b]+1;
            ret=max(ret,hash[arr[i]]);
        }
        return ret;
    }
};