Before revision: We give an algorithm, which given an n-vertex graph G and an integer k, in time 2^{O(k)} n either outputs a tree decomposition of G of width at most 2k + 1 or determines that the treewidth of G is larger than k  ]  .
After revision: We give an algorithm, which given an n-vertex graph G and an integer k, in time 2^{O(k)} n either outputs a tree decomposition of G of width at most 2k + 1 or determines that the treewidth of G is larger than k . This is the first 2-approximation algorithm for treewidth that is faster than the known exact algorithms. In particular, our algorithm improves upon both the previous best approximation ratio of 5 in time 2^{O(k) ] .