出现问题:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY
解决方案:
sudo gpg --keyserver keyserver.ubuntu.com --recv 648ACFD622F3D138
sudo gpg --export --armor 648ACFD622F3D138 | sudo apt-key add -
//648ACFD622F3D138 (这个公钥根据报错提示来写的)
继续执行
sudo apt-get update
THE END