#include <bits/stdc++.h>

#include <bits/stdc++.h> is an implementation file for a precompiled header.

From, software engineering perspective, it is a good idea to minimize the include. If you use it actually includes a lot of files, which your program may not need, thus increase both compile time and program size unnecessarily.

But in contests, using this file is a good idea, when you want to reduce the time wasted in doing chores; especially when your rank is time sensitive.

It works in most online judges,programming contest environments, including ACM-ICPC (Sub-Regionals, Regionals, and World Finals) and many online judges.

The disadvantages of it are that it

• increases the compilation time.
• uses an internal non-standard header file of the GNU C++ library, and so will not compile in MSVC, XCode, and many other compilers

Linux GCC 4.8.0 /bits/stdc++.h header definition.

// C++ includes used for precompiling -*- C++ -*-
     
    // Copyright (C) 2003-2013 Free Software Foundation, Inc.
    //
    // This file is part of the GNU ISO C++ Library.  This library is free
    // software; you can redistribute it and/or modify it under the
    // terms of the GNU General Public License as published by the
    // Free Software Foundation; either version 3, or (at your option)
    // any later version.
     
    // This library is distributed in the hope that it will be useful,
    // but WITHOUT ANY WARRANTY; without even the implied warranty of
    // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    // GNU General Public License for more details.
     
    // Under Section 7 of GPL version 3, you are granted additional
    // permissions described in the GCC Runtime Library Exception, version
    // 3.1, as published by the Free Software Foundation.
     
    // You should have received a copy of the GNU General Public License and
    // a copy of the GCC Runtime Library Exception along with this program;
    // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
    // <Licenses - GNU Project - Free Software Foundation>.
     
    /** @file stdc++.h
     *  This is an implementation file for a precompiled header.
     */
     
    // 17.4.1.2 Headers
     
    // C
    #ifndef _GLIBCXX_NO_ASSERT
    #include <cassert>
    #endif
    #include <cctype>
    #include <cerrno>
    #include <cfloat>
    #include <ciso646>
    #include <climits>
    #include <clocale>
    #include <cmath>
    #include <csetjmp>
    #include <csignal>
    #include <cstdarg>
    #include <cstddef>
    #include <cstdio>
    #include <cstdlib>
    #include <cstring>
    #include <ctime>
     
    #if __cplusplus >= 201103L
    #include <ccomplex>
    #include <cfenv>
    #include <cinttypes>
    #include <cstdalign>
    #include <cstdbool>
    #include <cstdint>
    #include <ctgmath>
    #include <cwchar>
    #include <cwctype>
    #endif
     
    // C++
    #include <algorithm>
    #include <bitset>
    #include <complex>
    #include <deque>
    #include <exception>
    #include <fstream>
    #include <functional>
    #include <iomanip>
    #include <ios>
    #include <iosfwd>
    #include <iostream>
    #include <istream>
    #include <iterator>
    #include <limits>
    #include <list>
    #include <locale>
    #include <map>
    #include <memory>
    #include <new>
    #include <numeric>
    #include <ostream>
    #include <queue>
    #include <set>
    #include <sstream>
    #include <stack>
    #include <stdexcept>
    #include <streambuf>
    #include <string>
    #include <typeinfo>
    #include <utility>
    #include <valarray>
    #include <vector>
     
    #if __cplusplus >= 201103L
    #include <array>
    #include <atomic>
    #include <chrono>
    #include <condition_variable>
    #include <forward_list>
    #include <future>
    #include <initializer_list>
    #include <mutex>
    #include <random>
    #include <ratio>
    #include <regex>
    #include <scoped_allocator>
    #include <system_error>
    #include <thread>
    #include <tuple>
    #include <typeindex>
    #include <type_traits>
    #include <unordered_map>
    #include <unordered_set>
    #endif