258.Add Digits

Tags: Easy Math

Links: https://leetcode.com/problems/add-digits/

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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?

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class Solution {
public:
    int addDigits(int num) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        return (num == 0) ? 0 : (num - 1) % 9 + 1;
    }
};

1    1
2    2
3    3
4    4
5    5
6    6
7    7
8    8    
9    9    
10    1
11    2
12    3    
13    4
14    5
15    6
16    7
17    8
18    9
19    1
20    2

找规律，当数字为0的时候需要特判。