Add Digits Problem & Solution
Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.
See the add digits problem on LeetCode.
C++ Solution
The runtime complexity of this algorithm is $O(1)$.
#pragma GCC optimize("Ofast")
static const int _=[](){std::ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();
class Solution {
public:
int addDigits(int num) {
if (num == 0) {
return 0;
}
// https://en.wikipedia.org/wiki/Digital_root
return 1 + (num - 1) % 9;
}
};