LeetCode 258: Add Digits

Meta Data

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Difficulty: easy First Attempt: 2022-08-22

• Submissions: Runtime Error ×2 → Accepted ×4

Approach

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Simulate: keep summing digits until a single digit remains (Math Template 5). For the O(1) follow-up: a number and its digit sum agree modulo 9 (since 10 ≡ 1 (mod 9)), so the final digit is n mod 9, except that multiples of 9 give 9 rather than 0: 0 if n == 0 else 1 + (n - 1) % 9.

My Attempts

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The first two used sum(list(str(num))), adding characters (TypeError), and attempt 1 also misnamed the parameter nums. Converting to int first passed; In 2025 I rewrote it in Python and in C++. Every time I simulated rather than using the formula.

Solution

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

Complexity

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Time O(1), space O(1).

Mistakes to Avoid

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• sum(list(str(n))) adds characters; convert to int first.
• The formula must handle 0 and multiples of 9.

Interview Explanation

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Simulation sums digits repeatedly; in O(1), a number and its digit sum are congruent mod 9, so the answer is 1 + (n - 1) % 9, with 0 for n = 0.