add ex 020 in cpp
add ex 020 in cpp

Mlieou/lXXtcode,Mlieou/oj_solutions,Mlieou/oj_solutions,Mlieou/oj_solutions,Mlieou/leetcode_python,Mlieou/lXXtcode,Mlieou/lXXtcode,Mlieou/oj_solutions,Mlieou/lXXtcode