LeetCode——Add Digits

LeetCode——Add Digits

Question

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?

Hint:

A naive implementation of the above process is trivial. Could you come up with other methods?

What are all the possible results?

How do they occur, periodically or randomly?

You may find this Wikipedia article useful.

Solution

十位数字+个位数字

Answer

class Solution {
public:
    int addDigits(int num) {
        int de;
        int re;
        while(num / 10 > 0) {
            de = num / 10;
            re = num % 10;
            num = de + re;
        }

        return num;
    }
};

这个问题叫做digital root. 有一个公式： n - 9 * [(n - 1) / 9]. 其中‘[]’ 表示下取整。