freopen("fanoinput.txt","r",stdin);

This line of code is used to redirect the standard input of a C++ program to a file named "fanoinput.txt".

The first parameter "fanoinput.txt" is the name of the file that will be used as the new standard input for the program.

The second parameter "r" stands for "read" and indicates that the file will be opened for reading.

The third parameter "stdin" is a predefined file pointer that represents the standard input stream. By redirecting the standard input to a file, the program will read input from that file instead of the keyboard.