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copied!<p>By default, <code>cin</code> is synchronized with stdio, which causes it to avoid any input buffering. If you add this to the top of your main, you should see much better performance:</p> <pre><code>std::ios_base::sync_with_stdio(false); </code></pre> <p>Normally, when an input stream is buffered, instead of reading one character at a time, the stream will be read in larger chunks. This reduces the number of system calls, which are typically relatively expensive. However, since the <code>FILE*</code> based <code>stdio</code> and <code>iostreams</code> often have separate implementations and therefore separate buffers, this could lead to a problem if both were used together. For example:</p> <pre><code>int myvalue1; cin >> myvalue1; int myvalue2; scanf("%d",&myvalue2); </code></pre> <p>If more input was read by <code>cin</code> than it actually needed, then the second integer value wouldn't be available for the <code>scanf</code> function, which has its own independent buffer. This would lead to unexpected results.</p> <p>To avoid this, by default, streams are synchronized with <code>stdio</code>. One common way to achieve this is to have <code>cin</code> read each character one at a time as needed using <code>stdio</code> functions. Unfortunately, this introduces a lot of overhead. For small amounts of input, this isn't a big problem, but when you are reading millions of lines, the performance penalty is significant.</p> <p>Fortunately, the library designers decided that you should also be able to disable this feature to get improved performance if you knew what you were doing, so they provided the <a href="http://en.cppreference.com/w/cpp/io/ios_base/sync_with_stdio" rel="noreferrer"><code>sync_with_stdio</code></a> method.</p>

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