错误信息

$ git push -u origin master
To github.com:liuzeyu12a/Servlet_Login.git
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to 'git@github.com:liuzeyu12a/Servlet_Login.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

解决方案：
首先是你的项目中有和和历史不符的东西
Push rejected: Push to origin/master was rejected
推拒绝：这是推送被拒绝
直接是解决办法，直接打开你要上传代码的文件夹位置鼠标右键git Bash Here然后直接下面两行命令解决问题

git pull origin master --allow-unrelated-histories
git push -u origin master -f