From 71146c8cf042659211c46bba610382634f4a1923 Mon Sep 17 00:00:00 2001
From: Gidijala uday srinu <55196913+udaysrinu@users.noreply.github.com>
Date: Tue, 6 Oct 2020 11:52:20 +0530
Subject: [PATCH] Create longest arithmetic subsequence

longest arithemetic subsequence leetcode
---
 C++/longest arithmetic subsequence | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)
 create mode 100644 C++/longest arithmetic subsequence

diff --git a/C++/longest arithmetic subsequence b/C++/longest arithmetic subsequence
new file mode 100644
index 00000000..4b6df6b5
--- /dev/null
+++ b/C++/longest arithmetic subsequence	
@@ -0,0 +1,23 @@
+class Solution {
+public:
+    int longestArithSeqLength(vector<int>& A) {
+         int n = A.size();
+    if(n<3)
+        return n;
+    vector<vector<int>> dp(n,vector<int>(n,-1));
+    int res=2;
+    unordered_map<int,int>mp;
+    for(int i=0;i<n;i++){
+        for(int j=i+1;j<n;j++){
+            dp[i][j]=2;
+            int need=2*A[i]-A[j];
+            if(mp.find(need)!=mp.end()){
+                dp[i][j]=max(dp[i][j],dp[mp[need]][i]+1);
+            }
+            res=max(res,dp[i][j]);
+        }
+        mp[A[i]]=i;   
+    }
+    return res;
+    }
+};