Computation-intensive　applications　offloading　is　challenging,　especially　in　the　designated　regions　where　communication　infrastructure　is　absent　or　compromised.　In　this　paper,　we　present　a　Space-Air-Ground　integrated　Mobile　Edge　Computing　(SAGiMEC)　system　for　these　regions　to　provide　quality　computational　services,　where　the　unmanned　aerial　vehicles　(UAVs)　act　as　in-fight　edge　servers　to　provide　low-latency　edge　computing　and　the　satellite　provides　resident　cloud　computing.　A　joint　optimization　problem　is　formulated　considering　UAV　deployment,　ground　device　(GD)　access,　and　computation　offloading　to　minimize　the　system　average　response　latency.　To　cope　with　the　problem＇s　complexity,　we　propose　a　Particle　Swarm　Optimization　(PSO)　and　Greedy　Strategy　(GS)-based　algorithm　(PSO　&　GS)　to　obtain　an　approximate　optimal　solution.　Extensive　simulations　validate　the　convergence　of　the　proposed　algorithm.　Numerical　results　show　that　the　proposed　approach　has　excellent　convergence,　and　the　system　average　response　latency　is　about　0.65x-0.85x　that　of　the　benchmark　algorithm.