页面置换算法的实现。里面包含了 LRU ,FIFO, 简单CLOCK，改进的CLOCK,OPT，随机算法这几种算法，然后根据情况选择算法。 -Page replacement algorithm. Which contains the LRU, FIFO, simple and CLOCK to improved CLOCK, OPT, random selection algorithm algorithm for these types of algorithms, and then depending on the circumstances.