there are two files, A.txt and B.txt.
there are 3000w rows of data in A.txt. The contents separated by spaces between id and username are as follows:

id   usernmae
1    zhangsan
2    lisi
......

there are 3000w rows of data in B.txt. The contents separated by spaces between id and realname are as follows:

id    realname
1     
2     
.....

how to extract the username from A.txt and append it to B.txt through id matching, and calculate the time complexity.