Untitled

class Solution {
public:
    int longestArithSeqLength(vector<int>& nums) {
        int n = nums.size(), ans = 0;
        vector<vector<int>> dp(1002, vector<int> (1002, 1));
        for(int i=n-1;i>=0;i--) {
            for(int j=i+1;j<n;j++) {
                int diff = nums[j] - nums[i] + 500;
                dp[i][diff] = 1 + dp[j][diff];
                ans = max(ans, dp[j][diff]); 
            }
        }
        return ans+1;
    }
};