“…It is shown in [40] that the complexity of this algorithm is O(f max /f min N ) in order to achieve an accuracy that is independent of the variation of f (x). In the fast sweeping method [5,55,47,54,27,28,53,38,39,52], Gauss-Seidel iterations with alternating orderings is combined with upwind finite differences. In contrast to the fast marching method, the fast sweeping method follows the causality along characteristics in a parallel way; i.e., all characteristics are divided into a finite number of groups according to their directions and each Gauss-Seidel iteration with a specific sweeping ordering covers a group of characteristics simultaneously; no heap-sort is needed.…”