看ubuntu默认的源太慢，就换了一波阿里的源，结果安装g++的时候疯狂出问题，不断提示需要依赖包，手动安装那个依赖包吧，又需要另一个依赖，贼烦，最后才意识到是源的问题，换成科大的就好了。。。
科大源的地址==》https://lug.ustc.edu.cn/repogen/
修改方法：
sudo mv /etc/apt/sources.list /etc/apt/sources.list.bak # 备份一下旧的源配置
sudo vim /etc/apt/sources.list
然后把科大源贴上去就行了。我是Ubuntu的，所以也就是下面的这些：
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
## Not recommended
# deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
# deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse保存退出后，更新一下系统，就可以安装了
sudo apt-get upgrade
sudo apt-get update
sudo apt-get install g++
安装完g++，gcc也会自动安上。查看一下版本信息确认安装完毕：
g++ –version
gcc –version
搞定收工