std::cout << std::endl : 插入换行并刷新缓存区 （flush the buffer）
std::cout << "\n" : 插入换行
其中关于 std::endl，C++ Primer 中是这样介绍的：
“endl, which is a special value called a manipulator. Writing endl has the effect of ending the current line and flushing the buffer associated with that device. Flushing the buffer ensures that all the output the program has generated so far is actually written to the output stream, rather than sitting in memory waiting to be written.”
-- Stanley B. Lippman. “C++ Primer, Fifth Edition (Jason Arnold‘s Library).”
