A faster FPTAS for knapsack problem with cardinality constraint

Abstract

We study the K-item knapsack problem (i.e., 1.5-dimensional knapsack problem), a generalization of the famous 0–1 knapsack problem (i.e., 1-dimensional knapsack problem) in which an upper bound K is imposed on the number of items selected. This problem is of fundamental importance and is known to have a broad range of applications in various fields. It is well known that, there is no fully polynomial time approximation scheme (FPTAS) for the d-dimensional knapsack problem when d≥2, unless P = NP. While the K-item knapsack problem is known to admit an FPTAS, the complexity of all existing FPTASs has a high dependency on the cardinality bound K and approximation error ɛ, which could result in inefficiencies especially when K and ɛ−1 increase. The current best results are due to Mastrolilli and Hutter (2006), in which two schemes are presented exhibiting a space–time tradeoff-one scheme with time complexity O(n+Kz2/ɛ2) and space complexity O(n+z3/ɛ), and another scheme that requires a run-time of O(n+(Kz2+z4)/ɛ2) but only needs O(n+z2/ɛ) space, where z=min{K,1/ɛ}.In this paper we close the space–time tradeoff exhibited in Mastrolilli and Hutter (2006) by designing a new FPTAS with a running time of O˜(n+z2/ɛ2), while simultaneously reaching a space complexity11O˜ notation hides terms poly-logarithmic in n and 1/ɛ. of O(n+z2/ɛ). Our scheme provides O˜(K) and O(z) improvements on the state-of-the-art algorithms in time and space complexity respectively, and is the first scheme that achieves a running time that is independent of the cardinality bound K (up to logarithmic factors) under fixed ɛ. Another salient feature of our algorithm is that it is the first FPTAS that achieves better time and space complexity bounds than the very first standard FPTAS over all parameter regimes.