[leetcode_258]Add Digits

I couldn’t figure out the O(1) approach, so here’s the simulation method: Repeatedly sum all the digits until the sum is less than 10.

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class Solution {
public:
    int addDigits(int num) {
        int sum = 0;
        while(num > 0)
        {
            sum += num % 10;
            num /= 10;
        }
        if (sum >= 10)
        {
            return addDigits(sum);
        }
        else
        {
            return sum;
        }
    }
};