LongestArithmeticSubseqWithDiff.cpp

1218. Longest Arithmetic Subsequence of Given Difference

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

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

 

Example 1:

Input: arr = [1,2,3,4], difference = 1
Output: 4
Explanation: The longest arithmetic subsequence is [1,2,3,4].
Example 2:

Input: arr = [1,3,5,7], difference = 1
Output: 1
Explanation: The longest arithmetic subsequence is any single element.
Example 3:

Input: arr = [1,5,7,8,5,3,4,2,1], difference = -2
Output: 4
Explanation: The longest arithmetic subsequence is [7,5,3,1].
  
  
  Solutions Given Below 
  
  class Solution {
public:
    int longestSubsequence(vector<int>& arr, int difference) {
        
        unordered_map<int , int>dp;
           int ans = 0;

        for(int i = 0;i<arr.size();i++){
            int temp = arr[i] - difference;
           
           int tempAns = 0;
          
          //to check answer in alredy exits in temp or not 
            if(dp.count(temp))
              tempAns  = dp[temp];
        //current ans update

        dp[arr[i]] =1+  tempAns;

       //update ans
        ans = max(ans ,dp[arr[i]]);
        }

        return ans;
    }
};