classSolution { public: intlongestArithSeqLength(vector<int>& A){ unordered_set<int> a; int result = 0; for (int i = 0; i < A.size(); i++) { a.insert(A[i]); } for (int i = 0; i < A.size()-1; i++) { for (int j = i+1; j < A.size(); j++) { int d = A[j] - A[i];
result = max(result, 2); int temp = A[j] + d; if (a.count(temp) == 0) continue; int temp_count = 2;
for (int k = j+1; k < A.size(); k++) { if (A[k] == temp) { temp_count++; temp += d; } } result = max(temp_count, result); } } return result; } };