c++中快读

c++中是自带输入的（废话），目前c++中有两种输入方法：

int main(){
    int n;
    cin>>n;
    return 0;
}

这种方法的优点是代码简洁，适合初学者。可是它太慢了。（有一次一道题我用cin结果输入就花了1s多）；

int main(){
    int n;
    scanf("%d",n);
    return 0;
}

这种方法相比cin快多了！可是有些毒瘤会超时。。。

当然，cin可以用std::ios::sync_with_stdio(false)来解决它输入慢的问题。

但是在noip中会爆零。因为noip只能用freopen,而freopen是stdio包含的，而std::ios::sync_with_stdio用了就不能用stdio。

手写快速读入的速度

首先声明一点：快读不是万能的，有些毒瘤会卡read。但，懂总比不懂好。

快读的原理是getchar()比cin和scanf快。所以，快读就是把输入的字符转为int。

inline long long read(){
     long long ans=0;
     char last=' ',ch=getchar();
     while(ch<'0' || ch>'9')last=ch,ch=getchar();
     while(ch>='0' && ch<='9')ans=ans*10+ch-'0',ch=getchar();
     if(last=='-')ans=-ans;
     return ans;
}

__int128的读写

__int128是c++中最大整型变量，存储范围为-2^127~2^127-1

但是__int128变量使用cin和scanf会报错：

tempCodeRunnerFile.cpp: In function 'int main()':
tempCodeRunnerFile.cpp:5:8: error: no match for 'operator>>' (operand types are 'std::istream' {aka 'std::basic_istream<char>'} and '__int128')
     cin>>n;
     ~~~^~~
In file included from C:/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/sstream:38,
                 from C:/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/complex:45,
                 from C:/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/ccomplex:39,
                 from C:/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/x86_64-w64-mingw32/bits/stdc++.h:52,
                 from tempCodeRunnerFile.cpp:1:
C:/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/istream:120:7: note: candidate: 'std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__istream_type& (*)(std::basic_istream<_CharT, _Traits>::__istream_type&)) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>]' <near match>        
       operator>>(__istream_type& (*__pf)(__istream_type&))
       ^~~~~~~~
C:/mingw64/lib/gcc/x86_64-w64-mingw32/8.1.0/include/c++/istream:120:7: note:   conversion of argument 1 would be ill-formed:
tempCodeRunnerFile.cpp:5:10: error: invalid conversion from '__int128' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&)' {aka 'std::basic_istream<char>& (*)(std::basic_istream<char>&)'} [-fpermissive]
     cin>>n;

这时候，只能用快读才能输入输出。。。（我也是因为这才了解快读的）

a+b problem with __int128:

#include <bits/stdc++.h>
using namespace std;
inline __int128 read(){
    __int128 x=0,f=1;
    char ch=getchar();
    while(ch<'0'||ch>'9'){
        if(ch=='-')
            f=-1;
        ch=getchar();
    }
    while(ch>='0'&&ch<='9'){
        x=x*10+ch-'0';
        ch=getchar();
    }
    return x*f;
}
inline void print(__int128 x){
    if(x<0){
        putchar('-');
        x=-x;
    }
    if(x>9)
        print(x/10);
    putchar(x%10+'0');
}
int main(void){
    __int128 a = read();
    __int128 b = read();
    print(a + b);
    cout<<endl;
    return 0;
}

p.s:noip 不可用__int128

这篇就结束了，下次再见。