Longest Arithmetic Progression

Given an array called A[] of sorted integers having no duplicates, find the length of the Longest Arithmetic Progression (LLAP) in it.

• Time : O(n^2)
• Space : O(n^2)

class Solution{
    unordered_map<int, int> table;
    unordered_map<int, int> dp[1000];
    int helper(int n, int d, int pos) {
        if(dp[pos].count(d)) return dp[pos][d];
        dp[pos][d] = 1;
        if(table.count(n + d))
            dp[pos][d] += helper(n + d, d, table[n + d]);
        return dp[pos][d];
    }
public:
    int lengthOfLongestAP(int A[], int n) {
        if(n == 1) return 1;
        table.clear();
        int res = 0;
        for(int i = n - 1; i >= 0; i--) {
            dp[i].clear();
            table[A[i]] = i;
            for(int j = i + 1; j < n; j++) {
                res = max(res, helper(A[i], A[j] - A[i], i));
            }
        }

        return res;
    }
};