  We present approximation algorithms with O(n^3) processing time for the
minimum vertex and edge guard problems in simple polygons. It is improved from
previous O(n^4) time algorithms of Ghosh. For simple polygon, there are O(n^3)
visibility regions, thus any approximation algorithm for the set covering
problem with approximation ratio of log(n) can be used for the approximation of
n vertex and edge guard problems with O(n^3) visibility sequence. We prove that
the visibility of all points in simple polygons is guaranteed by covering
O(n^2) sinks from vertices and edges : It comes to O(n^3) time bound.
