VERTEX-COVER: Given an undirected graph G=(V,E), find a minimum-size set of vertices
such that every edge has some vertex in the set.

References:

**2-approximation algorithm for vertex cover**

- Find a maximal matching and output the set of matched vertices

**thm 1.3 in text:*** The above algorithm is a 2-approximation algorithm for vertex cover.*

References:

- Chapter 1 of Approximation Algorithms by Vazirani.