Is Add Digits easy or hard?

Add Digits is classified as an Easy problem on LeetCode with a high acceptance rate. The simulation solution is straightforward, but recognizing the digital root formula demonstrates stronger mathematical insight.

How to solve Add Digits in O(1)?

Use the digital root property from number theory. If num is not zero, compute 1 + (num - 1) % 9. This formula directly returns the final digit produced by repeatedly summing digits, eliminating loops or recursion.

What is the best approach for Add Digits?

The best approach is the mathematical digital root formula: 1 + (num - 1) % 9 for num > 0, and 0 if num is 0. It computes the final single digit in constant time without repeatedly summing digits. This reduces the problem from multiple iterations to a single modulo operation.

What data structure is used in Add Digits?

No special data structure is required. The solution relies on arithmetic operations such as modulo and division to extract digits. The optimal version uses a number theory formula instead of any data structure.

What is the time complexity of Add Digits?

The optimal digital root approach runs in O(1) time and O(1) space because it uses a direct mathematical formula. A straightforward simulation that repeatedly sums digits takes O(d * k) time, where d is the number of digits and k is the number of iterations until a single digit remains.

Add Digits Python or Java solution approach?

In Python or Java, the simulation approach repeatedly extracts digits using % 10 and divides the number by 10 until one digit remains. The optimal implementation uses the digital root formula: return 0 if num == 0, otherwise return 1 + (num - 1) % 9.

Is Add Digits asked at Google, Amazon, or Meta?

Add Digits is a common easy-level screening problem used by companies like Google, Amazon, and Meta to test basic math reasoning and number manipulation. The problem often checks whether candidates recognize the digital root pattern rather than only implementing simulation.

Description Hints 4 Solution Video 5:40

258. Add Digits

Easy69.3% acceptance10 companies

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

Example 1:

Input: num = 38
Output: 2
Explanation: The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2 
Since 2 has only one digit, return it.

Example 2:

Input: num = 0
Output: 0

Constraints:

0 <= num <= 2³¹ - 1

Follow up: Could you do it without any loop/recursion in O(1) runtime?

Asked at 10 companies

Pega Adobe Amazon Microsoft Bloomberg

Google Uber

Infosys Visa Meta

Topics

Hints open one at a time

Hint 1

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

Hint 2

What are all the possible results?

Hint 3

How do they occur, periodically or randomly?

Hint 4

You may find this Wikipedia article useful.

5:40

Still stuck?Read 3 approaches, or watch a walkthrough.

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