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

Example:

Input: 38

Output: 2  Explanation: The process is like: 3 + 8 = 11, 1 + 1 = 2.               Since 2 has only one digit, return it. Follow up: Could you do it without any loop/recursion in O(1) runtime?

//常规操作public static int addDigits(int num) {    int result = num;    while(result >=10){         int sum = result;         result = 0;         while(sum>0){             result+=sum%10;             sum = sum/10;         }    }    return result;}

 

//看到别人这么写的  怎么推算的 不想动脑啦public int addDigits2(int num) {    if (num == 0){        return 0;    }    if (num % 9 == 0){        return 9;    }    else {        return num % 9;    }}

git:https://github.com/woshiyexinjie/leetcode-xin