看代码运行速度一下从48ms提升到24ms

解析
1. Lambda
首先先说一下这个语法

static const auto io_sync_off = {
… …
}();
这个乍一看很像是函数，但是前后又是[]又是()的，还没函数名没返回值的，感觉又不太像，所以就去请教了一下别人，得到答案是Lambda捕获。

Lambda表达式是C++11引入的特性，是一种描述函数对象的机制，它的主要应用是描述某些具有简单行为的函数。Lambda也可以称为匿名函数。

static const auto function() {
… …
}
static const auto io_sync_off = function();

2.std::ios::sync_with_stdio(false);
函数std :: ios_base :: sync_with_stdio的解释是

Sets whether the standard C++ streams are synchronized to the standard C streams after each input/output operation.
设置在每次输入/输出操作后标准C ++流是否与标准C流同步

随后又看到了一段关于std :: cin的解释

The global objects std::cin and std::wcin control input from a stream buffer of implementation-defined type (derived from std::streambuf), associated with the standard C input stream stdin.

These objects are guaranteed to be constructed before the first constructor of a static object is called and they are guaranteed to outlive the last destructor of a static object, so that it is always possible to read from std::cin in user code.

Unless sync_with_stdio(false) has been issued, it is safe to concurrently access these objects from multiple threads for both formatted and unformatted input.

Once std::cin is constructed, std::cin.tie() returns &std::cout, and likewise, std::wcin.tie() returns &std::wcout. This means that any formatted input operation on std::cin forces a call to std::cout.flush() if any characters are pending for output.

现在大概明白了一些，因为C++中的std :: cin和std :: cout为了兼容C，保证在代码中同时出现std :: cin和scanf或std :: cout和printf时输出不发生混乱，所以C++用一个流缓冲区来同步C的标准流。通过std :: ios_base :: sync_with_stdio函数可以解除这种同步，让std :: cin和std :: cout不再经过缓冲区，自然就节省了许多时间。

3. std::cin.tie(nullptr);
函数std :: ios :: tie的解释是

Get/set tied stream
The first form (1) returns a pointer to the tied output stream.

The second form (2) ties the object to tiestr and returns a pointer to the stream tied before the call, if any.

The tied stream is an output stream object which is flushed before each i/o operation in this stream object.

C++11
By default, the standard narrow streams cin and cerr are tied to cout, and their wide character counterparts (wcin and wcerr) to wcout. Library implementations may also tie clog and wclog.

这个现在看起来就比较容易理解了，因为std :: cin默认是与std :: cout绑定的，所以每次操作的时候（也就是调用”<<”或者”>>”）都要刷新（调用flush），这样增加了IO的负担，通过tie(nullptr)来解除std :: cin和std :: cout之间的绑定，来降低IO的负担使效率提升。