Digit Sum

Problem

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit, and return it.

Examples

digitSum(99)
// returns 9
// 99 becomes 9 + 9 which equals 18
// 18 then reduces to 1 + 8 which is 9

digitSum(10)
// returns 1
// 10 is 1 + 0, resulting in 1.

digitSum(12345)
// returns 6
// 12345 becomes 1 + 2 + 3 + 4 + 5, totaling 15
// 15 then simplifies to 1 + 5, giving 6