This paper investigates the subcarrier and power allocation for the downlink of a multicarrier non-orthogonal multiple access (MC-NOMA) system. A three-step algorithm is proposed to deal with the sum rate maximization problem. First, we assume each user could use all the subcarriers si- multaneously and apply the synchronous iterative waterfilling algorithm (SIWA) to obtain a power vector for each user. Second, subcarriers are assigned to users by a heuristic greedy method based on the achieved power allocation result of Step 1. Third, SIWA is used once again to further improve the system performance with the obtained subcarrier assignment result of Step 2. In Step 3, where the number of multiplexed users is no more than two, the convergence of SIWA is proved analytically. Numerical results show that the proposed resource allocation algorithm achieves comparable performance to an existing near- optimal solution but with much lower time complexity.