Is my friend cheating?

• Time : $O(n)$
• Space : $O(n)$

#include <vector>
using namespace std;
class RemovedNumbers
{
public:
    static vector<vector<long long>> removNb(long long n) {
        long long sum = n * (n + 1) / 2;
        vector<vector<long long>> res;
        for(long long a = 1; a <= n; a++) {
            long long b = (sum - a) / (a + 1);
            if(a * b + a + b == sum and 1 <= b and b <= n and b != a) res.push_back({a,b});
        }
        return res;
    }
};