Gauss - Correct

int solve(vector<vector<ldouble>> &A, vector<ldouble> &ans) {
    int rows = A.size();
    int cols = A[0].size() - 1;

    int row = 0;
    vector<int> where(cols, -1);

    for (int col = 0; col < cols; col++) {
        int chosen = row;
        while (chosen < rows && !sign(A[chosen][col])) chosen++;
        if (chosen >= rows) continue;

        where[col] = row;
        A[row].swap(A[chosen]);

        for (int i = 0; i < rows; i++)
            if (i != row) {
                ldouble factor = A[i][col] / A[row][col];
                for (int j = col; j <= cols; j++)
                    A[i][j] -= A[row][j] * factor;
            }

        row++;
    }

    bool freedom = false;
    ans.assign(cols, 0.0L);

    for (int i = 0; i < cols; i++) {
        if (where[i] == -1) freedom = true;
        else ans[i] = A[where[i]][cols] / A[where[i]][i];
    }

    for (int i = 0; i < rows; i++) {
        ldouble sum = 0.0L;
        for (int j = 0; j < cols; j++)
            sum += ans[j] * A[i][j];

        if (sign(sum - A[i][cols]))
            return 0;
    }

    return freedom ? INF : 1;
}