1.sudo apt install build-essential

 build-essential : Depends: g++ (>= 4:7.2) but it is not going to be installed
                   Depends: dpkg-dev (>= 1.17.11) but it is not going to be installed

2.sudo apt install g++

g++ : Depends: g++-7 (>= 7.3.0-12~) but it is not going to be installed

3.sudo apt install g++-7

g++-7 : Depends: gcc-7-base (= 7.3.0-16ubuntu3) but 7.4.0-1ubuntu1~18.04.1 is to be installed
         Depends: gcc-7 (= 7.3.0-16ubuntu3) but 7.4.0-1ubuntu1~18.04.1 is to be installed
         Depends: libstdc++-7-dev (= 7.3.0-16ubuntu3) but it is not going to be installed
4.sudo add-apt-repository ppa:jonathonf/gcc  (ps.如果想删除  cd /etc/apt/sources.list.d ，sudo rm 对应的)

5.sudo apt-get update

6.sudo apt install build-essential

build-essential : Depends: dpkg-dev (>= 1.17.11) but it is not going to be installed
7.sudo apt install dpkg-dev

dpkg-dev : Depends: libdpkg-perl (= 1.19.0.5ubuntu2) but 1.19.0.5ubuntu2.1 is to be installed
            Recommends: build-essential but it is not going to be installed
            Recommends: fakeroot
            Recommends: libalgorithm-merge-perl but it is not going to be installed

8.sudo apt-get remove libdpkg-perl

9.sudo apt install build-essential

success!!!!