Add Digits

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

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

第一步：当num<10的时候，直接返回num。

第二步：当num>10的时候，把每一位加一遍，得出一个数字s。

第三步：把s代入第一步，第二步再去算

public int addDigits(int num) {    if (num < 10)        return num;    int s = 0;    while (num > 0) {        s += num % 10;        num /= 10;    }    if (s < 10)        return s;    else        return addDigits(s);}