Problema
Add up the digits of n. If the result has more than one digit, add up its digits again. Repeat until a single digit remains.
Implement digital_root(n).
Input
n: A positive integer.
Output
Return the final single digit.
Restricciones
1 ≤ n ≤ 10¹⁸
Ejemplos
Ejemplo 1
Entrada:
n = 999
Salida esperada:
expected = 9Entrada:
long long n = 999;
Salida esperada:
long long expected = 9;Entrada:
long n = 999;
Salida esperada:
long expected = 9;Explicación
9 + 9 + 9 = 27, then 2 + 7 = 9.
Ejemplo 2
Entrada:
n = 38
Salida esperada:
expected = 2Entrada:
long long n = 38;
Salida esperada:
long long expected = 2;Entrada:
long n = 38;
Salida esperada:
long expected = 2;Explicación
3 + 8 = 11, then 1 + 1 = 2.
Cómo abordarlo
Inténtalo primero por tu cuenta y luego abre las pistas una a una.
Pista 1
Adding digits repeatedly eventually collapses the number to one digit.
Pista 2
You can simulate the repeated summing process until the value is less than 10.
Pista 3
Watch out for very large inputs; the process is simple, but repeated passes over the digits are the main work.
Complejidad esperada
O(d log n) time, O(1) extra space