Add Digits — LeetCode 258 Python Solution

The problem

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Example

Input

num = 38

Output

2

Explanation

The process is

Python solution

class Solution:
    def addDigits(self, num: int) -> int:
        return 0 if num == 0 else (num - 1) % 9 + 1

Complexity

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 258. Add Digits is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Math and Number Theory.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

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