Fast Input/Output in C++ for Competitive Programming

Fast input/output (I/O) is an important consideration in competitive programming, where time is often a critical factor in determining the performance of a solution. In C++, there are several techniques that can be used to improve the speed of I/O operations, which can help to make your solutions more efficient and competitive.

One of the most effective ways to improve the speed of I/O in C++ is to use the "fast I/O" technique, which involves bypassing the standard I/O streams (such as cin and cout) and directly reading and writing to the standard input and output streams (stdin and stdout). This can significantly improve the speed of I/O operations, especially on large inputs.

To use the fast I/O technique in C++, you will need to include the following header file:

#include <bits/stdc++.h>
      

This header file provides a number of useful functions and macros for competitive programming, including the fast I/O functions.

To read from the standard input stream (stdin), you can use the following function:

int readInt()
      {
          int x = 0, f = 1;
          char c = getchar();
          while (c < '0' || c > '9')
          {
              if (c == '-')
                  f = -1;
              c = getchar();
          }
          while (c >= '0' && c <= '9')
          {
              x = (x << 3) + (x << 1) + (c ^ '0');
              c = getchar();
          }
          return x * f;
      }
      

This function reads in a single integer from the standard input stream and returns it as an int. You can use this function to read in other data types as well, such as long long, double, and char.

To write to the standard output stream (stdout), you can use the following function:

void writeInt(int x)
{
    if (x < 0)
    {
        putchar('-');
        x = -x;
    }
    if (x > 9)
        writeInt(x / 10);
    putchar(x % 10 + '0');
}

This function writes a single integer to the standard output stream. Like the readInt function, you can use this function to write other data types as well.

To use the fast I/O functions in your code, you can simply call the readInt and writeInt functions as needed. For example:

int a = readInt();
int b = readInt();
int c = a + b;
writeInt(c);

This code reads in two integers from the standard input stream, adds them together, and writes the result to the standard output stream.

In addition to the fast I/O functions, there are a few other techniques that can be used to improve the speed of I/O in C++. One such technique is to use the scanf and printf functions, which are faster than the cin and cout streams. For example:

int a, b, c;
scanf("%d %d", &a, &b);
c = a + b;
printf("%d\n", c);

This code reads in two integers from the standard input stream and writes the result to the standard output stream.