C++为了兼容C，默认使iostream与stdio关联，使cin与scanf、cout和printf保持同步，保证混用过程中文件指针不混乱。
此方式会造成性能损失，导致使用cin/cout效率远远低于使用scanf/printf。若保证程序只是用一套指令，可取消同步：
std::ios::sync_with_stdio(false);
取消后使用cin/cout速度有明显提升，但注意不要混用iostream与stdio
std::cin.tie(0);
C++98：In terms of static initialization order, cin is guaranteed to be properly constructed and initialized no later than the first time an object of type ios_base::Init is constructed.
C++11：In terms of static initialization order, cin is guaranteed to be properly constructed and initialized no later than the first time an object of type ios_base::Init is constructed, with the inclusion of <iostream> counting as at least one initialization of such objects with static duration.
cin is tied to the standard output stream cout (see ios::tie), which indicates that cout‘s buffer is flushed (see ostream::flush) before each i/o operation performed on cin.
(来源:http://www.cplusplus.com/reference/iostream/cin/ 网址)
当传入参数为0时，实现cin与cout独立运行，不会等待，提高cin速度
全局域的通过静态常量初始化一个lambda的方式实现对此代码的优先调用
static const int _ = []() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
return 0;
}();