What does the 2 in a 2-approximation algorithm mean?
Typically, we use $\alpha &lt; 1$ for maximization problems, and $\alpha &gt; 1$ for minimization problems, where $\alpha$ is the approximation guarantee. So, a $2$-approximation algorithm returns a solution whose cost is at most twice the optimal. But as always, to be absolutely sure, go back to the definitions of the text you are reading (if a definition is not available, assume this).