The Modern C++ Challenge

51. Transforming a list of phone numbers

This problem is somewhat similar in some aspects to the previous one. However, instead of selecting phone numbers that start with a specified country code, we must transform each number so that they all start with that country code preceded by a +. There are several cases that must be considered:

• The phone number starts with a 0. That indicates a number without a country code. To modify the number to include the country code we must replace the 0 with the actual country code, preceded by +.
• The phone number starts with the country code. In this case, we just prepend + sign to the beginning.
• The phone number starts with + followed by the country code. In this case, the number is already in the expected ...