We present a near-tight analysis of the average query complexity -- `a la Nguyen and Onak [FOCS08] -- of the randomized greedy maximal matching algorithm, improving over the bound of Yoshida, Yamamoto and Ito [STOC09]. For any $n$-vertex graph of average degree $bar{d}$, this leads to the following sublinear-time algorithms for estimating the size of maximum matching and minimum vertex cover, all of which are provably time-optimal up to logarithmic factors: $bullet$ A multiplicative $(2+epsilon)$-approximation in $widetilde{O}(n/epsilon^2)$ time using adjacency list queries. This (nearly) matches an $Omega(n)$ time lower bound for any multiplicative approximation and is, notably, the first $O(1)$-approximation that runs in $o(n^{1.5})$ time. $bullet$ A $(2, epsilon n)$-approximation in $widetilde{O}((bar{d} + 1)/epsilon^2)$ time using adjacency list queries. This (nearly) matches an $Omega(bar{d}+1)$ lower bound of Parnas and Ron [TCS07] which holds for any $(O(1), epsilon n)$-approximation, and improves over the bounds of [Yoshida et al. STOC09; Onak et al. SODA12] and [Kapralov et al. SODA20]: The former two take at least quadratic time in the degree which can be as large as $Omega(n^2)$ and the latter obtains a much larger approximation. $bullet$ A $(2, epsilon n)$-approximation in $widetilde{O}(n/epsilon^3)$ time using adjacency matrix queries. This (nearly) matches an $Omega(n)$ time lower bound in this model and improves over the $widetilde{O}(nsqrt{n})$-time $(2, epsilon n)$-approximate algorithm of [Chen, Kannan, and Khanna ICALP20]. It also turns out that any non-trivial multiplicative approximation in the adjacency matrix model requires $Omega(n^2)$ time, so the additive $epsilon n$ error is necessary too. As immediate corollaries, we get improved sublinear time estimators for (variants of) TSP and an improved AMPC algorithm for maximal matching.
تحميل البحث