//自己做法
//大佬做法
function digital_root(n) {
console.log((n - 1),'1')
console.log((n - 1)% 9,'2')
console.log( (n - 1) % 9 + 1,'3');
}
digital_root(789)
digital_root(16)
=> 1 + 6
=> 7
digital_root(942)
=> 9 + 4 + 2
=> 15 ...
=> 1 + 5
=> 6
digital_root(132189)
=> 1 + 3 + 2 + 1 + 8 + 9
=> 24 ...
=> 2 + 4
=> 6
digital_root(493193)
=> 4 + 9 + 3 + 1 + 9 + 3
=> 29 ...
=> 2 + 9
=> 11 ...
=> 1 + 1
=> 2