Gengchun Xu | Searcharxiv

arXiv2017

Constant Query Time $(1 + ε)$-Approximate Distance Oracle for Planar Graphs

We give a $(1+ε)$-approximate distance oracle with $O(1)$ query time for an undirected planar graph $G$ with $n$ vertices and non-negative edge lengths. For $ε>0$ and any two vertices $u$ and $v$ in $G$, our oracle gives a distance $\tilde{d}(u,v)$ with stretch $(1+ε)$ in $O(1)$ time. The oracle has size $O(n\log n ((\log n)/ε+f(ε)))$ and pre-processing time $O(n\log n((\log^3 n)/ε^2+f(ε)))$, where $f(ε)=2^{O(1/ε)}$. This is the first $(1+ε)$-approximate distance oracle with $O(1)$ query time independent of $ε$ and the size and pre-processing time nearly linear in $n$, and improves the query time $O(1/ε)$ of previous $(1+ε)$-approximate distance oracle with size nearly linear in $n$.

cs.DS↗

arXiv2016

Near-Linear Time Constant-Factor Approximation Algorithm for Branch-Decomposition of Planar Graphs

We give an algorithm which for an input planar graph $G$ of $n$ vertices and integer $k$, in $\min\{O(n\log^3n),O(nk^2)\}$ time either constructs a branch-decomposition of $G$ with width at most $(2+δ)k$, $δ>0$ is a constant, or a $(k+1)\times \lceil{\frac{k+1}{2}\rceil}$ cylinder minor of $G$ implying $bw(G)>k$, $bw(G)$ is the branchwidth of $G$. This is the first $\tilde{O}(n)$ time constant-factor approximation for branchwidth/treewidth and largest grid/cylinder minors of planar graphs and improves the previous $\min\{O(n^{1+ε}),O(nk^2)\}$ ($ε>0$ is a constant) time constant-factor approximations. For a planar graph $G$ and $k=bw(G)$, a branch-decomposition of width at most $(2+δ)k$ and a $g\times \frac{g}{2}$ cylinder/grid minor with $g=\frac{k}β$, $β>2$ is constant, can be computed by our algorithm in $\min\{O(n\log^3n\log k),O(nk^2\log k)\}$ time.

cs.DS↗