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. ``` public class Solution { public int addDigits(int num) { if(num <10) { return num; } int t=0; while(num>=1){ t+=num%10; num=num/10; } return addDigits(t); } } public class Solution { public int addDigits(int num) { return (num - 1) % 9 + 1; } } ``` http://www.cnblogs.com/wxisme/