This paper proposes a novel systematic optimized comparison algorithm for LDPC codes with arbitrary code lengths. Based on the presented algorithm, we also design a corresponding decoder architecture which has a short comparison delay time of [log2(n-1)] and a very low comparator complexity of n× [log2 (n-1)] , where n is the input number to CNU. Besides, the proposed design is general, systematic and flexible, which can be applied to arbitrary input number of a CNU. The designed CNU is favorable to the existing CNU designs which are non-systematically designed with either longer critical path delays or higher comparator counts than the proposed designs. The proposed design is particularly good for long code length cases, when it is impractical to do customized optimized designs by hand, due to high design complexity.