计算理论导引第二版，近似算法（多项式时间的最优化问题、最小顶点覆盖（VERTEX-COVER）近似算法）、密码学（密钥、公钥密码系统）(Computational theory leads the second edition, approximation algorithms (polynomial time optimization problems, minimal vertex covering (VERTEX-COVER) approximation algorithms), cryp