47-longest-arithmetic-progression.cpp

#include <bits/stdc++.h>
using namespace std;

 // } Driver Code Ends
//User function template for C++

class Solution{   
public:
    int lengthOfLongestAP(int A[], int n) 
    {
        if(n == 1 || n == 2)
            return n;
        
        vector<vector<int>> dp(n, vector<int>(10001, 1));
        int ans = 1;
        for(int i = 0; i < n; ++i)
        {
            for(int j = i + 1; j < n; ++j)
            {
                int diff = A[j] - A[i];
                dp[j][diff] = dp[i][diff] + 1;
                ans = max(ans, dp[j][diff]);
            }
        }
        return ans;
    }
};

// { Driver Code Starts.
int main() {
    int t;
    cin >> t;
    while (t--) {
        int n;
        cin >> n;
        int A[n];
        for (int i = 0; i < n; i++) {
            cin >> A[i];
        }
        Solution ob;
        auto ans = ob.lengthOfLongestAP(A, n);
        cout << ans << "\n";
    }
    return 0;
}  // } Driver Code Ends