Use redirection to facilitate program testing

When you do a question on online judge, the program usually needs to read a large number of input data for testing. It is troublesome to input the test data from the keyboard during each operation.

To save the trouble of keyboard input, freopen is often used for input and output redirection. However, you need to comment them out after the test is complete. We can use the self-directed operation system to complete the process. For example, the program compiled is test.exe, and the test data is stored in in.txt. We can perform input redirection as follows:
Test <in.txt
Read the input data from in.txt. You can also perform output redirection at the same time:
Test <in.txt> out.txt
Read the input from in.txt and write the output to the out.txt file.
In this way, if you want to input data from the keyboard, you can directly press test. If you want to read data from a file, you can use redirection.