leetcode 258. Add Digits

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258. Add Digits

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Question

Total Accepted: 68661 Total Submissions: 142851 Difficulty: Easy

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.

Follow up:
Could you do it without any loop/recursion in O(1) runtime?

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Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

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思维题，找找规律，详见代码：

class Solution {
public:
    int addDigits(int num) {
        if(num <= 9){
            return num;
        }
        else{
            if(num % 9 == 0){
                return 9;
            }
            else{
                return num % 9;
            }
        }
    }
};