add and convert number...

add

https://leetcode.com/problems/add-digits/#/description
https://leetcode.com/problems/sum-of-two-integers/#/description
https://leetcode.com/problems/add-strings/#/description
https://leetcode.com/problems/add-binary/#/description

convert

https://leetcode.com/problems/base-7/#/description
https://leetcode.com/problems/convert-a-number-to-hexadecimal/#/description
https://leetcode.com/problems/roman-to-integer/#/description