编写程序计算 PI 值
要求：
① 采用至少两种 线程分配方法；
② 使用时间函数统计每个线程的计算时间，对比分析哪种分配方法更优；
③ 程序调试及运行，输出运行结果。(Write programs to calculate PI values
Requirements:
1.Use at least two kinds of thread allocation methods;
2.Use time function to calculate the computation time of each thread, then compare and analyse which distribution method is better;
3.Debug and run programs, finally output the running result.)