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

Abstract

We give constant-factor approximation algorithms for branch-decomposition of planar graphs. Our main result is an algorithm which for an input planar graph $G$ of $n$ vertices and integer $k$, in $O(n\log ^4n)$ time either constructs a branch-decomposition of $G$ with width at most $(2+\delta )k$, $\delta >0$ is a constant, or a $(k+1)\times \lceil {\frac{k+1}{2}}\rceil$ cylinder minor of $G$ implying ${\mathrm{bw}}(G)>k$, ${\mathrm{bw}}(G)$ 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 $\min \{O(n^{1+\epsilon }),O(nk^3)\}$ ($\epsilon >0$ is a constant) time constant-factor approximations. For a planar graph $G$ and $k={\mathrm{bw}}(G)$, a branch-decomposition of width at most $(2+\delta )k$ and a $g\times \frac{g}{2}$ cylinder/grid minor with $g=\frac{k}{\beta }$, $\beta >2$ is constant, can be computed by our algorithm in $O(n\log ^4n\log k)$ time.